Remove old RULE privilege completely.

[pgsql.git] / doc / src / sgml / monitoring.sgml

<!-- doc/src/sgml/monitoring.sgml -->

<chapter id="monitoring">
 <title>Monitoring Database Activity</title>

 <indexterm zone="monitoring">
  <primary>monitoring</primary>
  <secondary>database activity</secondary>
 </indexterm>

 <indexterm zone="monitoring">
  <primary>database activity</primary>
  <secondary>monitoring</secondary>
 </indexterm>

 <para>
  A database administrator frequently wonders, <quote>What is the system
  doing right now?</quote>
  This chapter discusses how to find that out.
 </para>

  <para>
   Several tools are available for monitoring database activity and
   analyzing performance.  Most of this chapter is devoted to describing
   <productname>PostgreSQL</productname>'s cumulative statistics system,
   but one should not neglect regular Unix monitoring programs such as
   <command>ps</command>, <command>top</command>, <command>iostat</command>, and <command>vmstat</command>.
   Also, once one has identified a
   poorly-performing query, further investigation might be needed using
   <productname>PostgreSQL</productname>'s <link linkend="sql-explain"><command>EXPLAIN</command></link> command.
   <xref linkend="using-explain"/> discusses <command>EXPLAIN</command>
   and other methods for understanding the behavior of an individual
   query.
  </para>

 <sect1 id="monitoring-ps">
  <title>Standard Unix Tools</title>

  <indexterm zone="monitoring-ps">
   <primary>ps</primary>
   <secondary>to monitor activity</secondary>
  </indexterm>

  <para>
   On most Unix platforms, <productname>PostgreSQL</productname> modifies its
   command title as reported by <command>ps</command>, so that individual server
   processes can readily be identified.  A sample display is

<screen>
$ ps auxww | grep ^postgres
postgres  15551  0.0  0.1  57536  7132 pts/0    S    18:02   0:00 postgres -i
postgres  15554  0.0  0.0  57536  1184 ?        Ss   18:02   0:00 postgres: background writer
postgres  15555  0.0  0.0  57536   916 ?        Ss   18:02   0:00 postgres: checkpointer
postgres  15556  0.0  0.0  57536   916 ?        Ss   18:02   0:00 postgres: walwriter
postgres  15557  0.0  0.0  58504  2244 ?        Ss   18:02   0:00 postgres: autovacuum launcher
postgres  15582  0.0  0.0  58772  3080 ?        Ss   18:04   0:00 postgres: joe runbug 127.0.0.1 idle
postgres  15606  0.0  0.0  58772  3052 ?        Ss   18:07   0:00 postgres: tgl regression [local] SELECT waiting
postgres  15610  0.0  0.0  58772  3056 ?        Ss   18:07   0:00 postgres: tgl regression [local] idle in transaction
</screen>

   (The appropriate invocation of <command>ps</command> varies across different
   platforms, as do the details of what is shown.  This example is from a
   recent Linux system.)  The first process listed here is the
   primary server process.  The command arguments
   shown for it are the same ones used when it was launched.  The next four
   processes are background worker processes automatically launched by the
   primary process.  (The <quote>autovacuum launcher</quote> process will not
   be present if you have set the system not to run autovacuum.)
   Each of the remaining
   processes is a server process handling one client connection.  Each such
   process sets its command line display in the form

<screen>
postgres: <replaceable>user</replaceable> <replaceable>database</replaceable> <replaceable>host</replaceable> <replaceable>activity</replaceable>
</screen>

  The user, database, and (client) host items remain the same for
  the life of the client connection, but the activity indicator changes.
  The activity can be <literal>idle</literal> (i.e., waiting for a client command),
  <literal>idle in transaction</literal> (waiting for client inside a <command>BEGIN</command> block),
  or a command type name such as <literal>SELECT</literal>.  Also,
  <literal>waiting</literal> is appended if the server process is presently waiting
  on a lock held by another session.  In the above example we can infer
  that process 15606 is waiting for process 15610 to complete its transaction
  and thereby release some lock.  (Process 15610 must be the blocker, because
  there is no other active session.  In more complicated cases it would be
  necessary to look into the
  <link linkend="view-pg-locks"><structname>pg_locks</structname></link>
  system view to determine who is blocking whom.)
  </para>

  <para>
   If <xref linkend="guc-cluster-name"/> has been configured the
   cluster name will also be shown in <command>ps</command> output:
<screen>
$ psql -c 'SHOW cluster_name'
 cluster_name
--------------
 server1
(1 row)

$ ps aux|grep server1
postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: server1: background writer
...
</screen>
  </para>

  <para>
   If you have turned off <xref linkend="guc-update-process-title"/> then the
   activity indicator is not updated; the process title is set only once
   when a new process is launched.  On some platforms this saves a measurable
   amount of per-command overhead;  on others it's insignificant.
  </para>

  <tip>
  <para>
  <productname>Solaris</productname> requires special handling. You must
  use <command>/usr/ucb/ps</command>, rather than
  <command>/bin/ps</command>. You also must use two <option>w</option>
  flags, not just one. In addition, your original invocation of the
  <command>postgres</command> command must have a shorter
  <command>ps</command> status display than that provided by each
  server process.  If you fail to do all three things, the <command>ps</command>
  output for each server process will be the original <command>postgres</command>
  command line.
  </para>
  </tip>
 </sect1>

 <sect1 id="monitoring-stats">
  <title>The Cumulative Statistics System</title>

  <indexterm zone="monitoring-stats">
   <primary>statistics</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname>'s <firstterm>cumulative statistics
   system</firstterm> supports collection and reporting of information about
   server activity.  Presently, accesses to tables and indexes in both
   disk-block and individual-row terms are counted.  The total number of rows
   in each table, and information about vacuum and analyze actions for each
   table are also counted.  If enabled, calls to user-defined functions and
   the total time spent in each one are counted as well.
  </para>

  <para>
   <productname>PostgreSQL</productname> also supports reporting dynamic
   information about exactly what is going on in the system right now, such as
   the exact command currently being executed by other server processes, and
   which other connections exist in the system.  This facility is independent
   of the cumulative statistics system.
  </para>

 <sect2 id="monitoring-stats-setup">
  <title>Statistics Collection Configuration</title>

  <para>
   Since collection of statistics adds some overhead to query execution,
   the system can be configured to collect or not collect information.
   This is controlled by configuration parameters that are normally set in
   <filename>postgresql.conf</filename>.  (See <xref linkend="runtime-config"/> for
   details about setting configuration parameters.)
  </para>

  <para>
   The parameter <xref linkend="guc-track-activities"/> enables monitoring
   of the current command being executed by any server process.
  </para>

  <para>
   The parameter <xref linkend="guc-track-counts"/> controls whether
   cumulative statistics are collected about table and index accesses.
  </para>

  <para>
   The parameter <xref linkend="guc-track-functions"/> enables tracking of
   usage of user-defined functions.
  </para>

  <para>
   The parameter <xref linkend="guc-track-io-timing"/> enables monitoring
   of block read, write, extend, and fsync times.
  </para>

  <para>
   The parameter <xref linkend="guc-track-wal-io-timing"/> enables monitoring
   of WAL write and fsync times.
  </para>

  <para>
   Normally these parameters are set in <filename>postgresql.conf</filename> so
   that they apply to all server processes, but it is possible to turn
   them on or off in individual sessions using the <xref
   linkend="sql-set"/> command. (To prevent
   ordinary users from hiding their activity from the administrator,
   only superusers are allowed to change these parameters with
   <command>SET</command>.)
  </para>

  <para>
   Cumulative statistics are collected in shared memory. Every
   <productname>PostgreSQL</productname> process collects statistics locally,
   then updates the shared data at appropriate intervals.  When a server,
   including a physical replica, shuts down cleanly, a permanent copy of the
   statistics data is stored in the <filename>pg_stat</filename> subdirectory,
   so that statistics can be retained across server restarts.  In contrast,
   when starting from an unclean shutdown (e.g., after an immediate shutdown,
   a server crash, starting from a base backup, and point-in-time recovery),
   all statistics counters are reset.
  </para>

 </sect2>

 <sect2 id="monitoring-stats-views">
  <title>Viewing Statistics</title>

  <para>
   Several predefined views, listed in <xref
   linkend="monitoring-stats-dynamic-views-table"/>, are available to show
   the current state of the system. There are also several other
   views, listed in <xref
   linkend="monitoring-stats-views-table"/>, available to show the accumulated
   statistics.  Alternatively, one can
   build custom views using the underlying cumulative statistics functions, as
   discussed in <xref linkend="monitoring-stats-functions"/>.
  </para>

  <para>
   When using the cumulative statistics views and functions to monitor
   collected data, it is important to realize that the information does not
   update instantaneously.  Each individual server process flushes out
   accumulated statistics to shared memory just before going idle, but not
   more frequently than once per <varname>PGSTAT_MIN_INTERVAL</varname>
   milliseconds (1 second unless altered while building the server); so a
   query or transaction still in progress does not affect the displayed totals
   and the displayed information lags behind actual activity.  However,
   current-query information collected by <varname>track_activities</varname>
   is always up-to-date.
  </para>

  <para>
   Another important point is that when a server process is asked to display
   any of the accumulated statistics, accessed values are cached until the end
   of its current transaction in the default configuration. So the statistics
   will show static information as long as you continue the current
   transaction. Similarly, information about the current queries of all
   sessions is collected when any such information is first requested within a
   transaction, and the same information will be displayed throughout the
   transaction. This is a feature, not a bug, because it allows you to perform
   several queries on the statistics and correlate the results without
   worrying that the numbers are changing underneath you.

   When analyzing statistics interactively, or with expensive queries, the
   time delta between accesses to individual statistics can lead to
   significant skew in the cached statistics. To minimize skew,
   <varname>stats_fetch_consistency</varname> can be set to
   <literal>snapshot</literal>, at the price of increased memory usage for
   caching not-needed statistics data.  Conversely, if it's known that
   statistics are only accessed once, caching accessed statistics is
   unnecessary and can be avoided by setting
   <varname>stats_fetch_consistency</varname> to <literal>none</literal>.

   You can invoke <function>pg_stat_clear_snapshot()</function> to discard the
   current transaction's statistics snapshot or cached values (if any).  The
   next use of statistical information will (when in snapshot mode) cause a
   new snapshot to be built or (when in cache mode) accessed statistics to be
   cached.
  </para>

  <para>
   A transaction can also see its own statistics (not yet flushed out to the
   shared memory statistics) in the views
   <structname>pg_stat_xact_all_tables</structname>,
   <structname>pg_stat_xact_sys_tables</structname>,
   <structname>pg_stat_xact_user_tables</structname>, and
   <structname>pg_stat_xact_user_functions</structname>.  These numbers do not act as
   stated above; instead they update continuously throughout the transaction.
  </para>

  <para>
   Some of the information in the dynamic statistics views shown in <xref
   linkend="monitoring-stats-dynamic-views-table"/> is security restricted.
   Ordinary users can only see all the information about their own sessions
   (sessions belonging to a role that they are a member of).  In rows about
   other sessions, many columns will be null.  Note, however, that the
   existence of a session and its general properties such as its sessions user
   and database are visible to all users.  Superusers and roles with privileges of
   built-in role <link linkend="predefined-role-pg-monitor"><literal>pg_read_all_stats</literal></link>
   can see all the information about all sessions.
  </para>

  <table id="monitoring-stats-dynamic-views-table">
   <title>Dynamic Statistics Views</title>

   <tgroup cols="2">
    <thead>
     <row>
      <entry>View Name</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>
       <structname>pg_stat_activity</structname>
       <indexterm><primary>pg_stat_activity</primary></indexterm>
      </entry>
      <entry>
       One row per server process, showing information related to
       the current activity of that process, such as state and current query.
       See <link linkend="monitoring-pg-stat-activity-view">
       <structname>pg_stat_activity</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_replication</structname><indexterm><primary>pg_stat_replication</primary></indexterm></entry>
      <entry>One row per WAL sender process, showing statistics about
       replication to that sender's connected standby server.
       See <link linkend="monitoring-pg-stat-replication-view">
       <structname>pg_stat_replication</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_wal_receiver</structname><indexterm><primary>pg_stat_wal_receiver</primary></indexterm></entry>
      <entry>Only one row, showing statistics about the WAL receiver from
       that receiver's connected server.
       See <link linkend="monitoring-pg-stat-wal-receiver-view">
       <structname>pg_stat_wal_receiver</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_recovery_prefetch</structname><indexterm><primary>pg_stat_recovery_prefetch</primary></indexterm></entry>
      <entry>Only one row, showing statistics about blocks prefetched during recovery.
       See <link linkend="monitoring-pg-stat-recovery-prefetch">
       <structname>pg_stat_recovery_prefetch</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_subscription</structname><indexterm><primary>pg_stat_subscription</primary></indexterm></entry>
      <entry>At least one row per subscription, showing information about
       the subscription workers.
       See <link linkend="monitoring-pg-stat-subscription">
       <structname>pg_stat_subscription</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_ssl</structname><indexterm><primary>pg_stat_ssl</primary></indexterm></entry>
      <entry>One row per connection (regular and replication), showing information about
       SSL used on this connection.
       See <link linkend="monitoring-pg-stat-ssl-view">
       <structname>pg_stat_ssl</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_gssapi</structname><indexterm><primary>pg_stat_gssapi</primary></indexterm></entry>
      <entry>One row per connection (regular and replication), showing information about
       GSSAPI authentication and encryption used on this connection.
       See <link linkend="monitoring-pg-stat-gssapi-view">
       <structname>pg_stat_gssapi</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_progress_analyze</structname><indexterm><primary>pg_stat_progress_analyze</primary></indexterm></entry>
      <entry>One row for each backend (including autovacuum worker processes) running
       <command>ANALYZE</command>, showing current progress.
       See <xref linkend="analyze-progress-reporting"/>.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_progress_create_index</structname><indexterm><primary>pg_stat_progress_create_index</primary></indexterm></entry>
      <entry>One row for each backend running <command>CREATE INDEX</command> or <command>REINDEX</command>, showing
      current progress.
      See <xref linkend="create-index-progress-reporting"/>.
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_progress_vacuum</structname><indexterm><primary>pg_stat_progress_vacuum</primary></indexterm></entry>
      <entry>One row for each backend (including autovacuum worker processes) running
       <command>VACUUM</command>, showing current progress.
       See <xref linkend="vacuum-progress-reporting"/>.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_progress_cluster</structname><indexterm><primary>pg_stat_progress_cluster</primary></indexterm></entry>
      <entry>One row for each backend running
       <command>CLUSTER</command> or <command>VACUUM FULL</command>, showing current progress.
       See <xref linkend="cluster-progress-reporting"/>.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_progress_basebackup</structname><indexterm><primary>pg_stat_progress_basebackup</primary></indexterm></entry>
      <entry>One row for each WAL sender process streaming a base backup,
       showing current progress.
       See <xref linkend="basebackup-progress-reporting"/>.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_progress_copy</structname><indexterm><primary>pg_stat_progress_copy</primary></indexterm></entry>
      <entry>One row for each backend running <command>COPY</command>, showing current progress.
       See <xref linkend="copy-progress-reporting"/>.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="monitoring-stats-views-table">
   <title>Collected Statistics Views</title>

   <tgroup cols="2">
    <thead>
     <row>
      <entry>View Name</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>

     <!-- everything related to global objects, alphabetically -->

     <row>
      <entry><structname>pg_stat_archiver</structname><indexterm><primary>pg_stat_archiver</primary></indexterm></entry>
      <entry>One row only, showing statistics about the
       WAL archiver process's activity. See
       <link linkend="monitoring-pg-stat-archiver-view">
       <structname>pg_stat_archiver</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_bgwriter</structname><indexterm><primary>pg_stat_bgwriter</primary></indexterm></entry>
      <entry>One row only, showing statistics about the
       background writer process's activity. See
       <link linkend="monitoring-pg-stat-bgwriter-view">
       <structname>pg_stat_bgwriter</structname></link> for details.
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_checkpointer</structname><indexterm><primary>pg_stat_checkpointer</primary></indexterm></entry>
      <entry>One row only, showing statistics about the
       checkpointer process's activity. See
       <link linkend="monitoring-pg-stat-checkpointer-view">
       <structname>pg_stat_checkpointer</structname></link> for details.
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_database</structname><indexterm><primary>pg_stat_database</primary></indexterm></entry>
      <entry>One row per database, showing database-wide statistics. See
       <link linkend="monitoring-pg-stat-database-view">
       <structname>pg_stat_database</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_database_conflicts</structname><indexterm><primary>pg_stat_database_conflicts</primary></indexterm></entry>
      <entry>
       One row per database, showing database-wide statistics about
       query cancels due to conflict with recovery on standby servers.
       See <link linkend="monitoring-pg-stat-database-conflicts-view">
       <structname>pg_stat_database_conflicts</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_io</structname><indexterm><primary>pg_stat_io</primary></indexterm></entry>
      <entry>
       One row for each combination of backend type, context, and target object
       containing cluster-wide I/O statistics.
       See <link linkend="monitoring-pg-stat-io-view">
       <structname>pg_stat_io</structname></link> for details.
     </entry>
     </row>

     <row>
      <entry><structname>pg_stat_replication_slots</structname><indexterm><primary>pg_stat_replication_slots</primary></indexterm></entry>
      <entry>One row per replication slot, showing statistics about the
       replication slot's usage. See
       <link linkend="monitoring-pg-stat-replication-slots-view">
       <structname>pg_stat_replication_slots</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_slru</structname><indexterm><primary>pg_stat_slru</primary></indexterm></entry>
      <entry>One row per SLRU, showing statistics of operations. See
       <link linkend="monitoring-pg-stat-slru-view">
       <structname>pg_stat_slru</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_subscription_stats</structname><indexterm><primary>pg_stat_subscription_stats</primary></indexterm></entry>
      <entry>One row per subscription, showing statistics about errors and conflicts.
      See <link linkend="monitoring-pg-stat-subscription-stats">
      <structname>pg_stat_subscription_stats</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_wal</structname><indexterm><primary>pg_stat_wal</primary></indexterm></entry>
      <entry>One row only, showing statistics about WAL activity. See
       <link linkend="monitoring-pg-stat-wal-view">
       <structname>pg_stat_wal</structname></link> for details.
      </entry>
     </row>

     <!-- all "stat" for schema objects, by "importance" -->

     <row>
      <entry><structname>pg_stat_all_tables</structname><indexterm><primary>pg_stat_all_tables</primary></indexterm></entry>
      <entry>
       One row for each table in the current database, showing statistics
       about accesses to that specific table.
       See <link linkend="monitoring-pg-stat-all-tables-view">
       <structname>pg_stat_all_tables</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_sys_tables</structname><indexterm><primary>pg_stat_sys_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_all_tables</structname>, except that only
      system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_user_tables</structname><indexterm><primary>pg_stat_user_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_all_tables</structname>, except that only user
      tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_all_tables</structname><indexterm><primary>pg_stat_xact_all_tables</primary></indexterm></entry>
      <entry>Similar to <structname>pg_stat_all_tables</structname>, but counts actions
      taken so far within the current transaction (which are <emphasis>not</emphasis>
      yet included in <structname>pg_stat_all_tables</structname> and related views).
      The columns for numbers of live and dead rows and vacuum and
      analyze actions are not present in this view.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_sys_tables</structname><indexterm><primary>pg_stat_xact_sys_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_xact_all_tables</structname>, except that only
      system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_user_tables</structname><indexterm><primary>pg_stat_xact_user_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_xact_all_tables</structname>, except that only
      user tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_all_indexes</structname><indexterm><primary>pg_stat_all_indexes</primary></indexterm></entry>
      <entry>
       One row for each index in the current database, showing statistics
       about accesses to that specific index.
       See <link linkend="monitoring-pg-stat-all-indexes-view">
       <structname>pg_stat_all_indexes</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_sys_indexes</structname><indexterm><primary>pg_stat_sys_indexes</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_all_indexes</structname>, except that only
      indexes on system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_user_indexes</structname><indexterm><primary>pg_stat_user_indexes</primary></indexterm></entry>
      <entry>Same as <structname>pg_stat_all_indexes</structname>, except that only
      indexes on user tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_stat_user_functions</structname><indexterm><primary>pg_stat_user_functions</primary></indexterm></entry>
      <entry>
       One row for each tracked function, showing statistics
       about executions of that function. See
       <link linkend="monitoring-pg-stat-user-functions-view">
       <structname>pg_stat_user_functions</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_stat_xact_user_functions</structname><indexterm><primary>pg_stat_xact_user_functions</primary></indexterm></entry>
      <entry>Similar to <structname>pg_stat_user_functions</structname>, but counts only
      calls during the current transaction (which are <emphasis>not</emphasis>
      yet included in <structname>pg_stat_user_functions</structname>).</entry>
     </row>

     <!-- all "statio" for schema objects, by "importance" -->

     <row>
      <entry><structname>pg_statio_all_tables</structname><indexterm><primary>pg_statio_all_tables</primary></indexterm></entry>
      <entry>
       One row for each table in the current database, showing statistics
       about I/O on that specific table.
       See <link linkend="monitoring-pg-statio-all-tables-view">
       <structname>pg_statio_all_tables</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_tables</structname><indexterm><primary>pg_statio_sys_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_tables</structname>, except that only
      system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_user_tables</structname><indexterm><primary>pg_statio_user_tables</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_tables</structname>, except that only
      user tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_all_indexes</structname><indexterm><primary>pg_statio_all_indexes</primary></indexterm></entry>
      <entry>
       One row for each index in the current database,
       showing statistics about I/O on that specific index.
       See <link linkend="monitoring-pg-statio-all-indexes-view">
       <structname>pg_statio_all_indexes</structname></link> for details.
      </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_indexes</structname><indexterm><primary>pg_statio_sys_indexes</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_indexes</structname>, except that only
      indexes on system tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_user_indexes</structname><indexterm><primary>pg_statio_user_indexes</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_indexes</structname>, except that only
      indexes on user tables are shown.</entry>
     </row>

     <row>
      <entry><structname>pg_statio_all_sequences</structname><indexterm><primary>pg_statio_all_sequences</primary></indexterm></entry>
     <entry>
       One row for each sequence in the current database,
       showing statistics about I/O on that specific sequence.
       See <link linkend="monitoring-pg-statio-all-sequences-view">
       <structname>pg_statio_all_sequences</structname></link> for details.
     </entry>
     </row>

     <row>
      <entry><structname>pg_statio_sys_sequences</structname><indexterm><primary>pg_statio_sys_sequences</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_sequences</structname>, except that only
      system sequences are shown.  (Presently, no system sequences are defined,
      so this view is always empty.)</entry>
     </row>

     <row>
      <entry><structname>pg_statio_user_sequences</structname><indexterm><primary>pg_statio_user_sequences</primary></indexterm></entry>
      <entry>Same as <structname>pg_statio_all_sequences</structname>, except that only
      user sequences are shown.</entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <para>
   The per-index statistics are particularly useful to determine which
   indexes are being used and how effective they are.
  </para>

  <para>
   The <structname>pg_stat_io</structname> and
   <structname>pg_statio_</structname> set of views are useful for determining
   the effectiveness of the buffer cache. They can be used to calculate a cache
   hit ratio. Note that while <productname>PostgreSQL</productname>'s I/O
   statistics capture most instances in which the kernel was invoked in order
   to perform I/O, they do not differentiate between data which had to be
   fetched from disk and that which already resided in the kernel page cache.
   Users are advised to use the <productname>PostgreSQL</productname>
   statistics views in combination with operating system utilities for a more
   complete picture of their database's I/O performance.
  </para>

 </sect2>

 <sect2 id="monitoring-pg-stat-activity-view">
  <title><structname>pg_stat_activity</structname></title>

  <indexterm>
   <primary>pg_stat_activity</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_activity</structname> view will have one row
   per server process, showing information related to
   the current activity of that process.
  </para>

  <table id="pg-stat-activity-view" xreflabel="pg_stat_activity">
   <title><structname>pg_stat_activity</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the database this backend is connected to
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datname</structfield> <type>name</type>
      </para>
      <para>
       Name of the database this backend is connected to
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of this backend
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>leader_pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of the parallel group leader if this process is a parallel
       query worker, or process ID of the leader apply worker if this process
       is a parallel apply worker.  <literal>NULL</literal> indicates that this
       process is a parallel group leader or leader apply worker, or does not
       participate in any parallel operation.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usesysid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the user logged into this backend
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usename</structfield> <type>name</type>
      </para>
      <para>
       Name of the user logged into this backend
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>application_name</structfield> <type>text</type>
      </para>
      <para>
       Name of the application that is connected
       to this backend
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>client_addr</structfield> <type>inet</type>
      </para>
      <para>
       IP address of the client connected to this backend.
       If this field is null, it indicates either that the client is
       connected via a Unix socket on the server machine or that this is an
       internal process such as autovacuum.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>client_hostname</structfield> <type>text</type>
      </para>
      <para>
       Host name of the connected client, as reported by a
       reverse DNS lookup of <structfield>client_addr</structfield>. This field will
       only be non-null for IP connections, and only when <xref linkend="guc-log-hostname"/> is enabled.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>client_port</structfield> <type>integer</type>
      </para>
      <para>
       TCP port number that the client is using for communication
       with this backend, or <literal>-1</literal> if a Unix socket is used.
       If this field is null, it indicates that this is an internal server process.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>backend_start</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time when this process was started.  For client backends,
       this is the time the client connected to the server.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>xact_start</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time when this process' current transaction was started, or null
       if no transaction is active. If the current
       query is the first of its transaction, this column is equal to the
       <structfield>query_start</structfield> column.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>query_start</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time when the currently active query was started, or if
       <structfield>state</structfield> is not <literal>active</literal>, when the last query
       was started
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>state_change</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time when the <structfield>state</structfield> was last changed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wait_event_type</structfield> <type>text</type>
      </para>
      <para>
       The type of event for which the backend is waiting, if any;
       otherwise NULL.  See <xref linkend="wait-event-table"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wait_event</structfield> <type>text</type>
      </para>
      <para>
       Wait event name if backend is currently waiting, otherwise NULL.
       See <xref linkend="wait-event-activity-table"/> through
       <xref linkend="wait-event-timeout-table"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>state</structfield> <type>text</type>
      </para>
      <para>
       Current overall state of this backend.
       Possible values are:
       <itemizedlist>
        <listitem>
        <para>
          <literal>active</literal>: The backend is executing a query.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>idle</literal>: The backend is waiting for a new client command.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>idle in transaction</literal>: The backend is in a transaction,
          but is not currently executing a query.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>idle in transaction (aborted)</literal>: This state is similar to
          <literal>idle in transaction</literal>, except one of the statements in
          the transaction caused an error.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>fastpath function call</literal>: The backend is executing a
          fast-path function.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>disabled</literal>: This state is reported if <xref linkend="guc-track-activities"/> is disabled in this backend.
         </para>
       </listitem>
       </itemizedlist>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>backend_xid</structfield> <type>xid</type>
      </para>
      <para>
       Top-level transaction identifier of this backend, if any;  see
       <xref linkend="transaction-id"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>backend_xmin</structfield> <type>xid</type>
      </para>
      <para>
       The current backend's <literal>xmin</literal> horizon.
      </para></entry>
     </row>

    <row>
     <entry role="catalog_table_entry"><para role="column_definition">
      <structfield>query_id</structfield> <type>bigint</type>
     </para>
     <para>
      Identifier of this backend's most recent query. If
      <structfield>state</structfield> is <literal>active</literal> this
      field shows the identifier of the currently executing query. In
      all other states, it shows the identifier of last query that was
      executed.  Query identifiers are not computed by default so this
      field will be null unless <xref linkend="guc-compute-query-id"/>
      parameter is enabled or a third-party module that computes query
      identifiers is configured.
     </para></entry>
    </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>query</structfield> <type>text</type>
      </para>
      <para>
       Text of this backend's most recent query. If
       <structfield>state</structfield> is <literal>active</literal> this field shows the
       currently executing query. In all other states, it shows the last query
       that was executed. By default the query text is truncated at 1024
       bytes; this value can be changed via the parameter
       <xref linkend="guc-track-activity-query-size"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>backend_type</structfield> <type>text</type>
      </para>
      <para>
       Type of current backend. Possible types are
       <literal>autovacuum launcher</literal>, <literal>autovacuum worker</literal>,
       <literal>logical replication launcher</literal>,
       <literal>logical replication worker</literal>,
       <literal>parallel worker</literal>, <literal>background writer</literal>,
       <literal>client backend</literal>, <literal>checkpointer</literal>,
       <literal>archiver</literal>, <literal>standalone backend</literal>,
       <literal>startup</literal>, <literal>walreceiver</literal>,
       <literal>walsender</literal>, <literal>walwriter</literal> and
       <literal>walsummarizer</literal>.
       In addition, background workers registered by extensions may have
       additional types.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <note>
   <para>
    The <structfield>wait_event</structfield> and <structfield>state</structfield> columns are
    independent.  If a backend is in the <literal>active</literal> state,
    it may or may not be <literal>waiting</literal> on some event.  If the state
    is <literal>active</literal> and <structfield>wait_event</structfield> is non-null, it
    means that a query is being executed, but is being blocked somewhere
    in the system.
   </para>
  </note>

  <table id="wait-event-table">
   <title>Wait Event Types</title>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>Wait Event Type</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><literal>Activity</literal></entry>
      <entry>The server process is idle.  This event type indicates a process
       waiting for activity in its main processing loop.
       <literal>wait_event</literal> will identify the specific wait point;
       see <xref linkend="wait-event-activity-table"/>.
      </entry>
     </row>
     <row>
      <entry><literal>BufferPin</literal></entry>
      <entry>The server process is waiting for exclusive access to
       a data buffer.  Buffer pin waits can be protracted if
       another process holds an open cursor that last read data from the
       buffer in question. See <xref linkend="wait-event-bufferpin-table"/>.
      </entry>
     </row>
     <row>
      <entry><literal>Client</literal></entry>
      <entry>The server process is waiting for activity on a socket
       connected to a user application.  Thus, the server expects something
       to happen that is independent of its internal processes.
       <literal>wait_event</literal> will identify the specific wait point;
       see <xref linkend="wait-event-client-table"/>.
      </entry>
     </row>
     <row>
      <entry><literal>Extension</literal></entry>
      <entry>The server process is waiting for some condition defined by an
       extension module.
       See <xref linkend="wait-event-extension-table"/>.
      </entry>
     </row>
     <row>
      <entry><literal>InjectionPoint</literal></entry>
      <entry>The server process is waiting for an injection point to reach an
       outcome defined in a test.  See
       <xref linkend="xfunc-addin-injection-points"/> for more details.  This
       type has no predefined wait points.
      </entry>
     </row>
     <row>
      <entry><literal>IO</literal></entry>
      <entry>The server process is waiting for an I/O operation to complete.
       <literal>wait_event</literal> will identify the specific wait point;
       see <xref linkend="wait-event-io-table"/>.
      </entry>
     </row>
     <row>
      <entry><literal>IPC</literal></entry>
      <entry>The server process is waiting for some interaction with
       another server process.  <literal>wait_event</literal> will
       identify the specific wait point;
       see <xref linkend="wait-event-ipc-table"/>.
      </entry>
     </row>
     <row>
      <entry><literal>Lock</literal></entry>
      <entry>The server process is waiting for a heavyweight lock.
       Heavyweight locks, also known as lock manager locks or simply locks,
       primarily protect SQL-visible objects such as tables.  However,
       they are also used to ensure mutual exclusion for certain internal
       operations such as relation extension.  <literal>wait_event</literal>
       will identify the type of lock awaited;
       see <xref linkend="wait-event-lock-table"/>.
      </entry>
     </row>
     <row>
      <entry><literal>LWLock</literal></entry>
      <entry> The server process is waiting for a lightweight lock.
       Most such locks protect a particular data structure in shared memory.
       <literal>wait_event</literal> will contain a name identifying the purpose
       of the lightweight lock.  (Some locks have specific names; others
       are part of a group of locks each with a similar purpose.)
       See <xref linkend="wait-event-lwlock-table"/>.
      </entry>
     </row>
     <row>
      <entry><literal>Timeout</literal></entry>
      <entry>The server process is waiting for a timeout
       to expire.  <literal>wait_event</literal> will identify the specific wait
       point; see <xref linkend="wait-event-timeout-table"/>.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  &wait_event_types;

   <para>
     Here are examples of how wait events can be viewed:

<programlisting>
SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL;
 pid  | wait_event_type | wait_event
------+-----------------+------------
 2540 | Lock            | relation
 6644 | LWLock          | ProcArray
(2 rows)
</programlisting>

<programlisting>
SELECT a.pid, a.wait_event, w.description
  FROM pg_stat_activity a JOIN
       pg_wait_events w ON (a.wait_event_type = w.type AND
                            a.wait_event = w.name)
  WHERE a.wait_event is NOT NULL and a.state = 'active';
-[ RECORD 1 ]------------------------------------------------------&zwsp;------------
pid         | 686674
wait_event  | WALInitSync
description | Waiting for a newly initialized WAL file to reach durable storage
</programlisting>
   </para>

   <note>
    <para>
     Extensions can add <literal>Extension</literal>,
     <literal>InjectionPoint</literal>. and <literal>LWLock</literal> events
     to the lists shown in <xref linkend="wait-event-extension-table"/> and
     <xref linkend="wait-event-lwlock-table"/>. In some cases, the name
     of an <literal>LWLock</literal> assigned by an extension will not be
     available in all server processes.  It might be reported as just
     <quote><literal>extension</literal></quote> rather than the
     extension-assigned name.
    </para>
   </note>
 </sect2>

 <sect2 id="monitoring-pg-stat-replication-view">
  <title><structname>pg_stat_replication</structname></title>

  <indexterm>
   <primary>pg_stat_replication</primary>
  </indexterm>

   <para>
   The <structname>pg_stat_replication</structname> view will contain one row
   per WAL sender process, showing statistics about replication to that
   sender's connected standby server.  Only directly connected standbys are
   listed; no information is available about downstream standby servers.
  </para>

  <table id="pg-stat-replication-view" xreflabel="pg_stat_replication">
   <title><structname>pg_stat_replication</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of a WAL sender process
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usesysid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the user logged into this WAL sender process
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>usename</structfield> <type>name</type>
      </para>
      <para>
       Name of the user logged into this WAL sender process
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>application_name</structfield> <type>text</type>
      </para>
      <para>
       Name of the application that is connected
       to this WAL sender
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>client_addr</structfield> <type>inet</type>
      </para>
      <para>
       IP address of the client connected to this WAL sender.
       If this field is null, it indicates that the client is
       connected via a Unix socket on the server machine.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>client_hostname</structfield> <type>text</type>
      </para>
      <para>
       Host name of the connected client, as reported by a
       reverse DNS lookup of <structfield>client_addr</structfield>. This field will
       only be non-null for IP connections, and only when <xref linkend="guc-log-hostname"/> is enabled.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>client_port</structfield> <type>integer</type>
      </para>
      <para>
       TCP port number that the client is using for communication
       with this WAL sender, or <literal>-1</literal> if a Unix socket is used
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>backend_start</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time when this process was started, i.e., when the
       client connected to this WAL sender
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>backend_xmin</structfield> <type>xid</type>
      </para>
      <para>
       This standby's <literal>xmin</literal> horizon reported
       by <xref linkend="guc-hot-standby-feedback"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>state</structfield> <type>text</type>
      </para>
      <para>
       Current WAL sender state.
       Possible values are:
       <itemizedlist>
        <listitem>
         <para>
          <literal>startup</literal>: This WAL sender is starting up.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>catchup</literal>: This WAL sender's connected standby is
          catching up with the primary.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>streaming</literal>: This WAL sender is streaming changes
          after its connected standby server has caught up with the primary.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>backup</literal>: This WAL sender is sending a backup.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>stopping</literal>: This WAL sender is stopping.
         </para>
        </listitem>
       </itemizedlist>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sent_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location sent on this connection
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>write_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location written to disk by this standby
       server
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>flush_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location flushed to disk by this standby
       server
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>replay_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location replayed into the database on this
       standby server
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>write_lag</structfield> <type>interval</type>
      </para>
      <para>
       Time elapsed between flushing recent WAL locally and receiving
       notification that this standby server has written it (but not yet
       flushed it or applied it).  This can be used to gauge the delay that
       <literal>synchronous_commit</literal> level
       <literal>remote_write</literal> incurred while committing if this
       server was configured as a synchronous standby.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>flush_lag</structfield> <type>interval</type>
      </para>
      <para>
       Time elapsed between flushing recent WAL locally and receiving
       notification that this standby server has written and flushed it
       (but not yet applied it).  This can be used to gauge the delay that
       <literal>synchronous_commit</literal> level
       <literal>on</literal> incurred while committing if this
       server was configured as a synchronous standby.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>replay_lag</structfield> <type>interval</type>
      </para>
      <para>
       Time elapsed between flushing recent WAL locally and receiving
       notification that this standby server has written, flushed and
       applied it.  This can be used to gauge the delay that
       <literal>synchronous_commit</literal> level
       <literal>remote_apply</literal> incurred while committing if this
       server was configured as a synchronous standby.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sync_priority</structfield> <type>integer</type>
      </para>
      <para>
       Priority of this standby server for being chosen as the
       synchronous standby in a priority-based synchronous replication.
       This has no effect in a quorum-based synchronous replication.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sync_state</structfield> <type>text</type>
      </para>
      <para>
       Synchronous state of this standby server.
       Possible values are:
       <itemizedlist>
        <listitem>
         <para>
          <literal>async</literal>: This standby server is asynchronous.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>potential</literal>: This standby server is now asynchronous,
          but can potentially become synchronous if one of current
          synchronous ones fails.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>sync</literal>: This standby server is synchronous.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>quorum</literal>: This standby server is considered as a candidate
          for quorum standbys.
         </para>
        </listitem>
       </itemizedlist>
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>reply_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Send time of last reply message received from standby server
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   The lag times reported in the <structname>pg_stat_replication</structname>
   view are measurements of the time taken for recent WAL to be written,
   flushed and replayed and for the sender to know about it.  These times
   represent the commit delay that was (or would have been) introduced by each
   synchronous commit level, if the remote server was configured as a
   synchronous standby.  For an asynchronous standby, the
   <structfield>replay_lag</structfield> column approximates the delay
   before recent transactions became visible to queries.  If the standby
   server has entirely caught up with the sending server and there is no more
   WAL activity, the most recently measured lag times will continue to be
   displayed for a short time and then show NULL.
  </para>

  <para>
   Lag times work automatically for physical replication. Logical decoding
   plugins may optionally emit tracking messages; if they do not, the tracking
   mechanism will simply display NULL lag.
  </para>

  <note>
   <para>
    The reported lag times are not predictions of how long it will take for
    the standby to catch up with the sending server assuming the current
    rate of replay.  Such a system would show similar times while new WAL is
    being generated, but would differ when the sender becomes idle.  In
    particular, when the standby has caught up completely,
    <structname>pg_stat_replication</structname> shows the time taken to
    write, flush and replay the most recent reported WAL location rather than
    zero as some users might expect.  This is consistent with the goal of
    measuring synchronous commit and transaction visibility delays for
    recent write transactions.
    To reduce confusion for users expecting a different model of lag, the
    lag columns revert to NULL after a short time on a fully replayed idle
    system. Monitoring systems should choose whether to represent this
    as missing data, zero or continue to display the last known value.
   </para>
  </note>

 </sect2>

 <sect2 id="monitoring-pg-stat-replication-slots-view">
  <title><structname>pg_stat_replication_slots</structname></title>

  <indexterm>
   <primary>pg_stat_replication_slots</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_replication_slots</structname> view will contain
   one row per logical replication slot, showing statistics about its usage.
  </para>

  <table id="pg-stat-replication-slots-view" xreflabel="pg_stat_replication_slots">
   <title><structname>pg_stat_replication_slots</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        Column Type
       </para>
       <para>
        Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>slot_name</structfield> <type>text</type>
       </para>
       <para>
        A unique, cluster-wide identifier for the replication slot
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>spill_txns</structfield> <type>bigint</type>
       </para>
       <para>
        Number of transactions spilled to disk once the memory used by
        logical decoding to decode changes from WAL has exceeded
        <literal>logical_decoding_work_mem</literal>. The counter gets
        incremented for both top-level transactions and subtransactions.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>spill_count</structfield> <type>bigint</type>
       </para>
       <para>
        Number of times transactions were spilled to disk while decoding
        changes from WAL for this slot. This counter is incremented each time
        a transaction is spilled, and the same transaction may be spilled
        multiple times.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>spill_bytes</structfield> <type>bigint</type>
       </para>
       <para>
        Amount of decoded transaction data spilled to disk while performing
        decoding of changes from WAL for this slot. This and other spill
        counters can be used to gauge the I/O which occurred during logical
        decoding and allow tuning <literal>logical_decoding_work_mem</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>stream_txns</structfield> <type>bigint</type>
       </para>
       <para>
        Number of in-progress transactions streamed to the decoding output
        plugin after the memory used by logical decoding to decode changes
        from WAL for this slot has exceeded
        <literal>logical_decoding_work_mem</literal>. Streaming only
        works with top-level transactions (subtransactions can't be streamed
        independently), so the counter is not incremented for subtransactions.
       </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>stream_count</structfield><type>bigint</type>
       </para>
       <para>
        Number of times in-progress transactions were streamed to the decoding
        output plugin while decoding changes from WAL for this slot. This
        counter is incremented each time a transaction is streamed, and the
        same transaction may be streamed multiple times.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>stream_bytes</structfield><type>bigint</type>
       </para>
       <para>
        Amount of transaction data decoded for streaming in-progress
        transactions to the decoding output plugin while decoding changes from
        WAL for this slot. This and other streaming counters for this slot can
        be used to tune <literal>logical_decoding_work_mem</literal>.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>total_txns</structfield> <type>bigint</type>
       </para>
       <para>
        Number of decoded transactions sent to the decoding output plugin for
        this slot. This counts top-level transactions only, and is not incremented
        for subtransactions. Note that this includes the transactions that are
        streamed and/or spilled.
       </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>total_bytes</structfield><type>bigint</type>
       </para>
       <para>
        Amount of transaction data decoded for sending transactions to the
        decoding output plugin while decoding changes from WAL for this slot.
        Note that this includes data that is streamed and/or spilled.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
       </para>
       <para>
        Time at which these statistics were last reset
       </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-wal-receiver-view">
  <title><structname>pg_stat_wal_receiver</structname></title>

  <indexterm>
   <primary>pg_stat_wal_receiver</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_wal_receiver</structname> view will contain only
   one row, showing statistics about the WAL receiver from that receiver's
   connected server.
  </para>

  <table id="pg-stat-wal-receiver-view" xreflabel="pg_stat_wal_receiver">
   <title><structname>pg_stat_wal_receiver</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of the WAL receiver process
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>status</structfield> <type>text</type>
      </para>
      <para>
       Activity status of the WAL receiver process
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>receive_start_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       First write-ahead log location used when WAL receiver is
       started
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>receive_start_tli</structfield> <type>integer</type>
      </para>
      <para>
       First timeline number used when WAL receiver is started
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>written_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location already received and written to disk,
       but not flushed. This should not be used for data integrity checks.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>flushed_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location already received and flushed to
       disk, the initial value of this field being the first log location used
       when WAL receiver is started
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>received_tli</structfield> <type>integer</type>
      </para>
      <para>
       Timeline number of last write-ahead log location received and
       flushed to disk, the initial value of this field being the timeline
       number of the first log location used when WAL receiver is started
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_msg_send_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Send time of last message received from origin WAL sender
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_msg_receipt_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Receipt time of last message received from origin WAL sender
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>latest_end_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location reported to origin WAL sender
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>latest_end_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time of last write-ahead log location reported to origin WAL sender
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>slot_name</structfield> <type>text</type>
      </para>
      <para>
       Replication slot name used by this WAL receiver
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sender_host</structfield> <type>text</type>
      </para>
      <para>
       Host of the <productname>PostgreSQL</productname> instance
       this WAL receiver is connected to. This can be a host name,
       an IP address, or a directory path if the connection is via
       Unix socket.  (The path case can be distinguished because it
       will always be an absolute path, beginning with <literal>/</literal>.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sender_port</structfield> <type>integer</type>
      </para>
      <para>
       Port number of the <productname>PostgreSQL</productname> instance
       this WAL receiver is connected to.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>conninfo</structfield> <type>text</type>
      </para>
      <para>
       Connection string used by this WAL receiver,
       with security-sensitive fields obfuscated.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-recovery-prefetch">
  <title><structname>pg_stat_recovery_prefetch</structname></title>

  <indexterm>
   <primary>pg_stat_recovery_prefetch</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_recovery_prefetch</structname> view will contain
   only one row.  The columns <structfield>wal_distance</structfield>,
   <structfield>block_distance</structfield> and
   <structfield>io_depth</structfield> show current values, and the
   other columns show cumulative counters that can be reset
   with the <function>pg_stat_reset_shared</function> function.
  </para>

  <table id="pg-stat-recovery-prefetch-view" xreflabel="pg_stat_recovery_prefetch">
   <title><structname>pg_stat_recovery_prefetch</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
       </para>
       <para>
        Time at which these statistics were last reset
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>prefetch</structfield> <type>bigint</type>
       </para>
       <para>
        Number of blocks prefetched because they were not in the buffer pool
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>hit</structfield> <type>bigint</type>
       </para>
       <para>
        Number of blocks not prefetched because they were already in the buffer pool
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>skip_init</structfield> <type>bigint</type>
       </para>
       <para>
        Number of blocks not prefetched because they would be zero-initialized
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>skip_new</structfield> <type>bigint</type>
       </para>
       <para>
        Number of blocks not prefetched because they didn't exist yet
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>skip_fpw</structfield> <type>bigint</type>
       </para>
       <para>
        Number of blocks not prefetched because a full page image was included in the WAL
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>skip_rep</structfield> <type>bigint</type>
       </para>
       <para>
        Number of blocks not prefetched because they were already recently prefetched
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>wal_distance</structfield> <type>int</type>
       </para>
       <para>
        How many bytes ahead the prefetcher is looking
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>block_distance</structfield> <type>int</type>
       </para>
       <para>
        How many blocks ahead the prefetcher is looking
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>io_depth</structfield> <type>int</type>
       </para>
       <para>
        How many prefetches have been initiated but are not yet known to have completed
       </para>
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-subscription">
  <title><structname>pg_stat_subscription</structname></title>

  <indexterm>
   <primary>pg_stat_subscription</primary>
  </indexterm>

  <table id="pg-stat-subscription" xreflabel="pg_stat_subscription">
   <title><structname>pg_stat_subscription</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>subid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the subscription
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>subname</structfield> <type>name</type>
      </para>
      <para>
       Name of the subscription
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>worker_type</structfield> <type>text</type>
      </para>
      <para>
       Type of the subscription worker process.  Possible types are
       <literal>apply</literal>, <literal>parallel apply</literal>, and
       <literal>table synchronization</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of the subscription worker process
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>leader_pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of the leader apply worker if this process is a parallel
       apply worker; NULL if this process is a leader apply worker or a table
       synchronization worker
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the relation that the worker is synchronizing; NULL for the
       leader apply worker and parallel apply workers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>received_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location received, the initial value of
       this field being 0; NULL for parallel apply workers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_msg_send_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Send time of last message received from origin WAL sender; NULL for
       parallel apply workers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_msg_receipt_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Receipt time of last message received from origin WAL sender; NULL for
       parallel apply workers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>latest_end_lsn</structfield> <type>pg_lsn</type>
      </para>
      <para>
       Last write-ahead log location reported to origin WAL sender; NULL for
       parallel apply workers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>latest_end_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time of last write-ahead log location reported to origin WAL
       sender; NULL for parallel apply workers
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-subscription-stats">
  <title><structname>pg_stat_subscription_stats</structname></title>

  <indexterm>
   <primary>pg_stat_subscription_stats</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_subscription_stats</structname> view will contain
   one row per subscription.
  </para>

  <table id="pg-stat-subscription-stats" xreflabel="pg_stat_subscription_stats">
   <title><structname>pg_stat_subscription_stats</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>subid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the subscription
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>subname</structfield> <type>name</type>
      </para>
      <para>
       Name of the subscription
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>apply_error_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times an error occurred while applying changes. Note that any
       conflict resulting in an apply error will be counted in both
       <literal>apply_error_count</literal> and the corresponding conflict
       count (e.g., <literal>confl_*</literal>).
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sync_error_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times an error occurred during the initial table
       synchronization
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_insert_exists</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times a row insertion violated a
       <literal>NOT DEFERRABLE</literal> unique constraint during the
       application of changes. See <xref linkend="conflict-insert-exists"/>
       for details about this conflict.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_update_origin_differs</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times an update was applied to a row that had been previously
       modified by another source during the application of changes. See
       <xref linkend="conflict-update-origin-differs"/> for details about this
       conflict.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_update_exists</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times that an updated row value violated a
       <literal>NOT DEFERRABLE</literal> unique constraint during the
       application of changes. See <xref linkend="conflict-update-exists"/>
       for details about this conflict.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_update_missing</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times the tuple to be updated was not found during the
       application of changes. See <xref linkend="conflict-update-missing"/>
       for details about this conflict.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_delete_origin_differs</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times a delete operation was applied to row that had been
       previously modified by another source during the application of changes.
       See <xref linkend="conflict-delete-origin-differs"/> for details about
       this conflict.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_delete_missing</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times the tuple to be deleted was not found during the application
       of changes. See <xref linkend="conflict-delete-missing"/> for details
       about this conflict.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time at which these statistics were last reset
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-ssl-view">
  <title><structname>pg_stat_ssl</structname></title>

  <indexterm>
   <primary>pg_stat_ssl</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_ssl</structname> view will contain one row per
   backend or WAL sender process, showing statistics about SSL usage on
   this connection. It can be joined to <structname>pg_stat_activity</structname>
   or <structname>pg_stat_replication</structname> on the
   <structfield>pid</structfield> column to get more details about the
   connection.
  </para>

  <table id="pg-stat-ssl-view" xreflabel="pg_stat_ssl">
   <title><structname>pg_stat_ssl</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of a backend or WAL sender process
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>ssl</structfield> <type>boolean</type>
      </para>
      <para>
       True if SSL is used on this connection
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>version</structfield> <type>text</type>
      </para>
      <para>
       Version of SSL in use, or NULL if SSL is not in use
       on this connection
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>cipher</structfield> <type>text</type>
      </para>
      <para>
       Name of SSL cipher in use, or NULL if SSL is not in use
       on this connection
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>bits</structfield> <type>integer</type>
      </para>
      <para>
       Number of bits in the encryption algorithm used, or NULL
       if SSL is not used on this connection
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>client_dn</structfield> <type>text</type>
      </para>
      <para>
       Distinguished Name (DN) field from the client certificate
       used, or NULL if no client certificate was supplied or if SSL
       is not in use on this connection. This field is truncated if the
       DN field is longer than <symbol>NAMEDATALEN</symbol> (64 characters
       in a standard build).
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>client_serial</structfield> <type>numeric</type>
      </para>
      <para>
       Serial number of the client certificate, or NULL if no client
       certificate was supplied or if SSL is not in use on this connection.  The
       combination of certificate serial number and certificate issuer uniquely
       identifies a certificate (unless the issuer erroneously reuses serial
       numbers).
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>issuer_dn</structfield> <type>text</type>
      </para>
      <para>
       DN of the issuer of the client certificate, or NULL if no client
       certificate was supplied or if SSL is not in use on this connection.
       This field is truncated like <structfield>client_dn</structfield>.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-gssapi-view">
  <title><structname>pg_stat_gssapi</structname></title>

  <indexterm>
   <primary>pg_stat_gssapi</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_gssapi</structname> view will contain one row per
   backend, showing information about GSSAPI usage on this connection. It can
   be joined to <structname>pg_stat_activity</structname> or
   <structname>pg_stat_replication</structname> on the
   <structfield>pid</structfield> column to get more details about the
   connection.
  </para>

  <table id="pg-stat-gssapi-view" xreflabel="pg_stat_gssapi">
   <title><structname>pg_stat_gssapi</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of a backend
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>gss_authenticated</structfield> <type>boolean</type>
      </para>
      <para>
       True if GSSAPI authentication was used for this connection
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>principal</structfield> <type>text</type>
      </para>
      <para>
       Principal used to authenticate this connection, or NULL
       if GSSAPI was not used to authenticate this connection.  This
       field is truncated if the principal is longer than
       <symbol>NAMEDATALEN</symbol> (64 characters in a standard build).
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>encrypted</structfield> <type>boolean</type>
      </para>
      <para>
       True if GSSAPI encryption is in use on this connection
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>credentials_delegated</structfield> <type>boolean</type>
      </para>
      <para>
       True if GSSAPI credentials were delegated on this connection.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-archiver-view">
  <title><structname>pg_stat_archiver</structname></title>

  <indexterm>
   <primary>pg_stat_archiver</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_archiver</structname> view will always have a
   single row, containing data about the archiver process of the cluster.
  </para>

  <table id="pg-stat-archiver-view" xreflabel="pg_stat_archiver">
   <title><structname>pg_stat_archiver</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>archived_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of WAL files that have been successfully archived
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_archived_wal</structfield> <type>text</type>
      </para>
      <para>
       Name of the WAL file most recently successfully archived
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_archived_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time of the most recent successful archive operation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>failed_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of failed attempts for archiving WAL files
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_failed_wal</structfield> <type>text</type>
      </para>
      <para>
       Name of the WAL file of the most recent failed archival operation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_failed_time</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time of the most recent failed archival operation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time at which these statistics were last reset
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
    Normally, WAL files are archived in order, oldest to newest, but that is
    not guaranteed, and does not hold under special circumstances like when
    promoting a standby or after crash recovery. Therefore it is not safe to
    assume that all files older than
    <structfield>last_archived_wal</structfield> have also been successfully
    archived.
  </para>
 </sect2>

 <sect2 id="monitoring-pg-stat-io-view">
  <title><structname>pg_stat_io</structname></title>

  <indexterm>
   <primary>pg_stat_io</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_io</structname> view will contain one row for each
   combination of backend type, target I/O object, and I/O context, showing
   cluster-wide I/O statistics. Combinations which do not make sense are
   omitted.
  </para>

  <para>
   Currently, I/O on relations (e.g. tables, indexes) is tracked. However,
   relation I/O which bypasses shared buffers (e.g. when moving a table from one
   tablespace to another) is currently not tracked.
  </para>

  <table id="pg-stat-io-view" xreflabel="pg_stat_io">
   <title><structname>pg_stat_io</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        Column Type
       </para>
       <para>
        Description
       </para>
      </entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>backend_type</structfield> <type>text</type>
       </para>
       <para>
        Type of backend (e.g. background worker, autovacuum worker). See <link
        linkend="monitoring-pg-stat-activity-view">
        <structname>pg_stat_activity</structname></link> for more information
        on <varname>backend_type</varname>s. Some
        <varname>backend_type</varname>s do not accumulate I/O operation
        statistics and will not be included in the view.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>object</structfield> <type>text</type>
       </para>
       <para>
        Target object of an I/O operation. Possible values are:
       <itemizedlist>
        <listitem>
         <para>
          <literal>relation</literal>: Permanent relations.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>temp relation</literal>: Temporary relations.
         </para>
        </listitem>
       </itemizedlist>
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>context</structfield> <type>text</type>
       </para>
       <para>
        The context of an I/O operation. Possible values are:
       </para>
       <itemizedlist>
        <listitem>
         <para>
          <literal>normal</literal>: The default or standard
          <varname>context</varname> for a type of I/O operation. For
          example, by default, relation data is read into and written out from
          shared buffers. Thus, reads and writes of relation data to and from
          shared buffers are tracked in <varname>context</varname>
          <literal>normal</literal>.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>vacuum</literal>: I/O operations performed outside of shared
          buffers while vacuuming and analyzing permanent relations. Temporary
          table vacuums use the same local buffer pool as other temporary table
          I/O operations and are tracked in <varname>context</varname>
          <literal>normal</literal>.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>bulkread</literal>: Certain large read I/O operations
          done outside of shared buffers, for example, a sequential scan of a
          large table.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>bulkwrite</literal>: Certain large write I/O operations
          done outside of shared buffers, such as <command>COPY</command>.
         </para>
        </listitem>
       </itemizedlist>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>reads</structfield> <type>bigint</type>
       </para>
       <para>
        Number of read operations, each of the size specified in
        <varname>op_bytes</varname>.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>read_time</structfield> <type>double precision</type>
       </para>
       <para>
        Time spent in read operations in milliseconds (if
        <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>writes</structfield> <type>bigint</type>
       </para>
       <para>
        Number of write operations, each of the size specified in
        <varname>op_bytes</varname>.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>write_time</structfield> <type>double precision</type>
       </para>
       <para>
        Time spent in write operations in milliseconds (if
        <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>writebacks</structfield> <type>bigint</type>
       </para>
       <para>
        Number of units of size <varname>op_bytes</varname> which the process
        requested the kernel write out to permanent storage.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>writeback_time</structfield> <type>double precision</type>
       </para>
       <para>
        Time spent in writeback operations in milliseconds (if
        <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero). This
        includes the time spent queueing write-out requests and, potentially,
        the time spent to write out the dirty data.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>extends</structfield> <type>bigint</type>
       </para>
       <para>
        Number of relation extend operations, each of the size specified in
        <varname>op_bytes</varname>.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>extend_time</structfield> <type>double precision</type>
       </para>
       <para>
        Time spent in extend operations in milliseconds (if
        <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>op_bytes</structfield> <type>bigint</type>
       </para>
       <para>
        The number of bytes per unit of I/O read, written, or extended.
       </para>
       <para>
        Relation data reads, writes, and extends are done in
        <varname>block_size</varname> units, derived from the build-time
        parameter <symbol>BLCKSZ</symbol>, which is <literal>8192</literal> by
        default.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>hits</structfield> <type>bigint</type>
       </para>
       <para>
        The number of times a desired block was found in a shared buffer.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>evictions</structfield> <type>bigint</type>
       </para>
       <para>
        Number of times a block has been written out from a shared or local
        buffer in order to make it available for another use.
       </para>
       <para>
        In <varname>context</varname> <literal>normal</literal>, this counts
        the number of times a block was evicted from a buffer and replaced with
        another block. In <varname>context</varname>s
        <literal>bulkwrite</literal>, <literal>bulkread</literal>, and
        <literal>vacuum</literal>, this counts the number of times a block was
        evicted from shared buffers in order to add the shared buffer to a
        separate, size-limited ring buffer for use in a bulk I/O operation.
        </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>reuses</structfield> <type>bigint</type>
       </para>
       <para>
        The number of times an existing buffer in a size-limited ring buffer
        outside of shared buffers was reused as part of an I/O operation in the
        <literal>bulkread</literal>, <literal>bulkwrite</literal>, or
        <literal>vacuum</literal> <varname>context</varname>s.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>fsyncs</structfield> <type>bigint</type>
       </para>
       <para>
        Number of <literal>fsync</literal> calls. These are only tracked in
        <varname>context</varname> <literal>normal</literal>.
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>fsync_time</structfield> <type>double precision</type>
       </para>
       <para>
        Time spent in fsync operations in milliseconds (if
        <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
       </para>
      </entry>
     </row>

     <row>
      <entry role="catalog_table_entry">
       <para role="column_definition">
        <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
       </para>
       <para>
        Time at which these statistics were last reset.
       </para>
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   Some backend types never perform I/O operations on some I/O objects and/or
   in some I/O contexts. These rows are omitted from the view. For example, the
   checkpointer does not checkpoint temporary tables, so there will be no rows
   for <varname>backend_type</varname> <literal>checkpointer</literal> and
   <varname>object</varname> <literal>temp relation</literal>.
  </para>

  <para>
   In addition, some I/O operations will never be performed either by certain
   backend types or on certain I/O objects and/or in certain I/O contexts.
   These cells will be NULL. For example, temporary tables are not
   <literal>fsync</literal>ed, so <varname>fsyncs</varname> will be NULL for
   <varname>object</varname> <literal>temp relation</literal>. Also, the
   background writer does not perform reads, so <varname>reads</varname> will
   be NULL in rows for <varname>backend_type</varname> <literal>background
   writer</literal>.
  </para>

  <para>
   <structname>pg_stat_io</structname> can be used to inform database tuning.
   For example:
   <itemizedlist>
    <listitem>
     <para>
      A high <varname>evictions</varname> count can indicate that shared
      buffers should be increased.
     </para>
    </listitem>
    <listitem>
     <para>
      Client backends rely on the checkpointer to ensure data is persisted to
      permanent storage. Large numbers of <varname>fsyncs</varname> by
      <literal>client backend</literal>s could indicate a misconfiguration of
      shared buffers or of the checkpointer. More information on configuring
      the checkpointer can be found in <xref linkend="wal-configuration"/>.
     </para>
    </listitem>
    <listitem>
     <para>
      Normally, client backends should be able to rely on auxiliary processes
      like the checkpointer and the background writer to write out dirty data
      as much as possible. Large numbers of writes by client backends could
      indicate a misconfiguration of shared buffers or of the checkpointer.
      More information on configuring the checkpointer can be found in <xref
      linkend="wal-configuration"/>.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <note>
   <para>
    Columns tracking I/O time will only be non-zero when
    <xref linkend="guc-track-io-timing"/> is enabled. The user should be
    careful when referencing these columns in combination with their
    corresponding I/O operations in case <varname>track_io_timing</varname>
    was not enabled for the entire time since the last stats reset.
   </para>
  </note>



 </sect2>

 <sect2 id="monitoring-pg-stat-bgwriter-view">
  <title><structname>pg_stat_bgwriter</structname></title>

  <indexterm>
   <primary>pg_stat_bgwriter</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_bgwriter</structname> view will always have a
   single row, containing data about the background writer of the cluster.
  </para>

  <table id="pg-stat-bgwriter-view" xreflabel="pg_stat_bgwriter">
   <title><structname>pg_stat_bgwriter</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>buffers_clean</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffers written by the background writer
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>maxwritten_clean</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times the background writer stopped a cleaning
       scan because it had written too many buffers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>buffers_alloc</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffers allocated
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time at which these statistics were last reset
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-checkpointer-view">
  <title><structname>pg_stat_checkpointer</structname></title>

  <indexterm>
   <primary>pg_stat_checkpointer</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_checkpointer</structname> view will always have a
   single row, containing data about the checkpointer process of the cluster.
  </para>

  <table id="pg-stat-checkpointer-view" xreflabel="pg_stat_checkpointer">
   <title><structname>pg_stat_checkpointer</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>num_timed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of scheduled checkpoints that have been performed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>num_requested</structfield> <type>bigint</type>
      </para>
      <para>
       Number of requested checkpoints that have been performed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>restartpoints_timed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of scheduled restartpoints due to timeout or after a failed attempt to perform it
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>restartpoints_req</structfield> <type>bigint</type>
      </para>
      <para>
       Number of requested restartpoints
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>restartpoints_done</structfield> <type>bigint</type>
      </para>
      <para>
       Number of restartpoints that have been performed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>write_time</structfield> <type>double precision</type>
      </para>
      <para>
       Total amount of time that has been spent in the portion of
       processing checkpoints and restartpoints where files are written to disk,
       in milliseconds
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sync_time</structfield> <type>double precision</type>
      </para>
      <para>
       Total amount of time that has been spent in the portion of
       processing checkpoints and restartpoints where files are synchronized to
       disk, in milliseconds
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>buffers_written</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffers written during checkpoints and restartpoints
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time at which these statistics were last reset
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-wal-view">
   <title><structname>pg_stat_wal</structname></title>

  <indexterm>
   <primary>pg_stat_wal</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_wal</structname> view will always have a
   single row, containing data about WAL activity of the cluster.
  </para>

  <table id="pg-stat-wal-view" xreflabel="pg_stat_wal">
   <title><structname>pg_stat_wal</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_records</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of WAL records generated
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_fpi</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of WAL full page images generated
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_bytes</structfield> <type>numeric</type>
      </para>
      <para>
       Total amount of WAL generated in bytes
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_buffers_full</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times WAL data was written to disk because WAL buffers became full
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_write</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times WAL buffers were written out to disk via
       <function>XLogWrite</function> request.
       See <xref linkend="wal-configuration"/> for more information about
       the internal WAL function <function>XLogWrite</function>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_sync</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times WAL files were synced to disk via
       <function>issue_xlog_fsync</function> request
       (if <xref linkend="guc-fsync"/> is <literal>on</literal> and
       <xref linkend="guc-wal-sync-method"/> is either
       <literal>fdatasync</literal>, <literal>fsync</literal> or
       <literal>fsync_writethrough</literal>, otherwise zero).
       See <xref linkend="wal-configuration"/> for more information about
       the internal WAL function <function>issue_xlog_fsync</function>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_write_time</structfield> <type>double precision</type>
      </para>
      <para>
       Total amount of time spent writing WAL buffers to disk via
       <function>XLogWrite</function> request, in milliseconds
       (if <xref linkend="guc-track-wal-io-timing"/> is enabled,
       otherwise zero).  This includes the sync time when
       <varname>wal_sync_method</varname> is either
       <literal>open_datasync</literal> or <literal>open_sync</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>wal_sync_time</structfield> <type>double precision</type>
      </para>
      <para>
       Total amount of time spent syncing WAL files to disk via
       <function>issue_xlog_fsync</function> request, in milliseconds
       (if <varname>track_wal_io_timing</varname> is enabled,
       <varname>fsync</varname> is <literal>on</literal>, and
       <varname>wal_sync_method</varname> is either
       <literal>fdatasync</literal>, <literal>fsync</literal> or
       <literal>fsync_writethrough</literal>, otherwise zero).
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time at which these statistics were last reset
      </para></entry>
     </row>
     </tbody>
   </tgroup>
  </table>

</sect2>

 <sect2 id="monitoring-pg-stat-database-view">
  <title><structname>pg_stat_database</structname></title>

  <indexterm>
   <primary>pg_stat_database</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_database</structname> view will contain one row
   for each database in the cluster, plus one for shared objects, showing
   database-wide statistics.
  </para>

  <table id="pg-stat-database-view" xreflabel="pg_stat_database">
   <title><structname>pg_stat_database</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datid</structfield> <type>oid</type>
      </para>
      <para>
       OID of this database, or 0 for objects belonging to a shared
       relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datname</structfield> <type>name</type>
      </para>
      <para>
       Name of this database, or <literal>NULL</literal> for shared
       objects.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>numbackends</structfield> <type>integer</type>
      </para>
      <para>
       Number of backends currently connected to this database, or
       <literal>NULL</literal> for shared objects.  This is the only column
       in this view that returns a value reflecting current state; all other
       columns return the accumulated values since the last reset.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>xact_commit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of transactions in this database that have been
       committed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>xact_rollback</structfield> <type>bigint</type>
      </para>
      <para>
       Number of transactions in this database that have been
       rolled back
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks read in this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_hit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times disk blocks were found already in the buffer
       cache, so that a read was not necessary (this only includes hits in the
       PostgreSQL buffer cache, not the operating system's file system cache)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tup_returned</structfield> <type>bigint</type>
      </para>
      <para>
       Number of live rows fetched by sequential scans and index entries returned by index scans in this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tup_fetched</structfield> <type>bigint</type>
      </para>
      <para>
       Number of live rows fetched by index scans in this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tup_inserted</structfield> <type>bigint</type>
      </para>
      <para>
       Number of rows inserted by queries in this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tup_updated</structfield> <type>bigint</type>
      </para>
      <para>
       Number of rows updated by queries in this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tup_deleted</structfield> <type>bigint</type>
      </para>
      <para>
       Number of rows deleted by queries in this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>conflicts</structfield> <type>bigint</type>
      </para>
      <para>
       Number of queries canceled due to conflicts with recovery
       in this database. (Conflicts occur only on standby servers; see
       <link linkend="monitoring-pg-stat-database-conflicts-view">
       <structname>pg_stat_database_conflicts</structname></link> for details.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>temp_files</structfield> <type>bigint</type>
      </para>
      <para>
       Number of temporary files created by queries in this database.
       All temporary files are counted, regardless of why the temporary file
       was created (e.g., sorting or hashing), and regardless of the
       <xref linkend="guc-log-temp-files"/> setting.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>temp_bytes</structfield> <type>bigint</type>
      </para>
      <para>
       Total amount of data written to temporary files by queries in
       this database. All temporary files are counted, regardless of why
       the temporary file was created, and
       regardless of the <xref linkend="guc-log-temp-files"/> setting.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>deadlocks</structfield> <type>bigint</type>
      </para>
      <para>
       Number of deadlocks detected in this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>checksum_failures</structfield> <type>bigint</type>
      </para>
      <para>
       Number of data page checksum failures detected in this
       database (or on a shared object), or NULL if data checksums are not
       enabled.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>checksum_last_failure</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time at which the last data page checksum failure was detected in
       this database (or on a shared object), or NULL if data checksums are not
       enabled.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blk_read_time</structfield> <type>double precision</type>
      </para>
      <para>
       Time spent reading data file blocks by backends in this database,
       in milliseconds (if <xref linkend="guc-track-io-timing"/> is enabled,
       otherwise zero)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blk_write_time</structfield> <type>double precision</type>
      </para>
      <para>
       Time spent writing data file blocks by backends in this database,
       in milliseconds (if <xref linkend="guc-track-io-timing"/> is enabled,
       otherwise zero)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>session_time</structfield> <type>double precision</type>
      </para>
      <para>
       Time spent by database sessions in this database, in milliseconds
       (note that statistics are only updated when the state of a session
       changes, so if sessions have been idle for a long time, this idle time
       won't be included)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>active_time</structfield> <type>double precision</type>
      </para>
      <para>
       Time spent executing SQL statements in this database, in milliseconds
       (this corresponds to the states <literal>active</literal> and
       <literal>fastpath function call</literal> in
       <link linkend="monitoring-pg-stat-activity-view">
       <structname>pg_stat_activity</structname></link>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idle_in_transaction_time</structfield> <type>double precision</type>
      </para>
      <para>
       Time spent idling while in a transaction in this database, in milliseconds
       (this corresponds to the states <literal>idle in transaction</literal> and
       <literal>idle in transaction (aborted)</literal> in
       <link linkend="monitoring-pg-stat-activity-view">
       <structname>pg_stat_activity</structname></link>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sessions</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of sessions established to this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sessions_abandoned</structfield> <type>bigint</type>
      </para>
      <para>
       Number of database sessions to this database that were terminated
       because connection to the client was lost
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sessions_fatal</structfield> <type>bigint</type>
      </para>
      <para>
       Number of database sessions to this database that were terminated
       by fatal errors
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sessions_killed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of database sessions to this database that were terminated
       by operator intervention
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time at which these statistics were last reset
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-database-conflicts-view">
  <title><structname>pg_stat_database_conflicts</structname></title>

  <indexterm>
   <primary>pg_stat_database_conflicts</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_database_conflicts</structname> view will contain
   one row per database, showing database-wide statistics about
   query cancels occurring due to conflicts with recovery on standby servers.
   This view will only contain information on standby servers, since
   conflicts do not occur on primary servers.
  </para>

  <table id="pg-stat-database-conflicts-view" xreflabel="pg_stat_database_conflicts">
   <title><structname>pg_stat_database_conflicts</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datid</structfield> <type>oid</type>
      </para>
      <para>
       OID of a database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datname</structfield> <type>name</type>
      </para>
      <para>
       Name of this database
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_tablespace</structfield> <type>bigint</type>
      </para>
      <para>
       Number of queries in this database that have been canceled due to
       dropped tablespaces
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_lock</structfield> <type>bigint</type>
      </para>
      <para>
       Number of queries in this database that have been canceled due to
       lock timeouts
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_snapshot</structfield> <type>bigint</type>
      </para>
      <para>
       Number of queries in this database that have been canceled due to
       old snapshots
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_bufferpin</structfield> <type>bigint</type>
      </para>
      <para>
       Number of queries in this database that have been canceled due to
       pinned buffers
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_deadlock</structfield> <type>bigint</type>
      </para>
      <para>
       Number of queries in this database that have been canceled due to
       deadlocks
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>confl_active_logicalslot</structfield> <type>bigint</type>
      </para>
      <para>
       Number of uses of logical slots in this database that have been
       canceled due to old snapshots or too low a <xref linkend="guc-wal-level"/>
       on the primary
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-all-tables-view">
  <title><structname>pg_stat_all_tables</structname></title>

  <indexterm>
   <primary>pg_stat_all_tables</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_all_tables</structname> view will contain
   one row for each table in the current database (including TOAST
   tables), showing statistics about accesses to that specific table. The
   <structname>pg_stat_user_tables</structname> and
   <structname>pg_stat_sys_tables</structname> views
   contain the same information,
   but filtered to only show user and system tables respectively.
  </para>

  <table id="pg-stat-all-tables-view" xreflabel="pg_stat_all_tables">
   <title><structname>pg_stat_all_tables</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of a table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
      </para>
      <para>
       Name of the schema that this table is in
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relname</structfield> <type>name</type>
      </para>
      <para>
       Name of this table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>seq_scan</structfield> <type>bigint</type>
      </para>
      <para>
       Number of sequential scans initiated on this table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_seq_scan</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       The time of the last sequential scan on this table, based on the
       most recent transaction stop time
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>seq_tup_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of live rows fetched by sequential scans
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_scan</structfield> <type>bigint</type>
      </para>
      <para>
       Number of index scans initiated on this table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_idx_scan</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       The time of the last index scan on this table, based on the
       most recent transaction stop time
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_tup_fetch</structfield> <type>bigint</type>
      </para>
      <para>
       Number of live rows fetched by index scans
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_tup_ins</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of rows inserted
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_tup_upd</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of rows updated.  (This includes row updates
       counted in <structfield>n_tup_hot_upd</structfield> and
       <structfield>n_tup_newpage_upd</structfield>, and remaining
       non-<acronym>HOT</acronym> updates.)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_tup_del</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of rows deleted
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_tup_hot_upd</structfield> <type>bigint</type>
      </para>
      <para>
       Number of rows <link linkend="storage-hot">HOT updated</link>.
       These are updates where no successor versions are required in
       indexes.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_tup_newpage_upd</structfield> <type>bigint</type>
      </para>
      <para>
       Number of rows updated where the successor version goes onto a
       <emphasis>new</emphasis> heap page, leaving behind an original
       version with a
       <link linkend="storage-tuple-layout"><structfield>t_ctid</structfield>
        field</link> that points to a different heap page.  These are
       always non-<acronym>HOT</acronym> updates.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_live_tup</structfield> <type>bigint</type>
      </para>
      <para>
       Estimated number of live rows
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_dead_tup</structfield> <type>bigint</type>
      </para>
      <para>
       Estimated number of dead rows
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_mod_since_analyze</structfield> <type>bigint</type>
      </para>
      <para>
       Estimated number of rows modified since this table was last analyzed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>n_ins_since_vacuum</structfield> <type>bigint</type>
      </para>
      <para>
       Estimated number of rows inserted since this table was last vacuumed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_vacuum</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Last time at which this table was manually vacuumed
       (not counting <command>VACUUM FULL</command>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_autovacuum</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Last time at which this table was vacuumed by the autovacuum
       daemon
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_analyze</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Last time at which this table was manually analyzed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_autoanalyze</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Last time at which this table was analyzed by the autovacuum
       daemon
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>vacuum_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times this table has been manually vacuumed
       (not counting <command>VACUUM FULL</command>)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>autovacuum_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times this table has been vacuumed by the autovacuum
       daemon
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>analyze_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times this table has been manually analyzed
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>autoanalyze_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times this table has been analyzed by the autovacuum
       daemon
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-all-indexes-view">
  <title><structname>pg_stat_all_indexes</structname></title>

  <indexterm>
   <primary>pg_stat_all_indexes</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_all_indexes</structname> view will contain
   one row for each index in the current database,
   showing statistics about accesses to that specific index. The
   <structname>pg_stat_user_indexes</structname> and
   <structname>pg_stat_sys_indexes</structname> views
   contain the same information,
   but filtered to only show user and system indexes respectively.
  </para>

  <table id="pg-stat-all-indexes-view" xreflabel="pg_stat_all_indexes">
   <title><structname>pg_stat_all_indexes</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the table for this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>indexrelid</structfield> <type>oid</type>
      </para>
      <para>
       OID of this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
      </para>
      <para>
       Name of the schema this index is in
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relname</structfield> <type>name</type>
      </para>
      <para>
       Name of the table for this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>indexrelname</structfield> <type>name</type>
      </para>
      <para>
       Name of this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_scan</structfield> <type>bigint</type>
      </para>
      <para>
       Number of index scans initiated on this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>last_idx_scan</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       The time of the last scan on this index, based on the
       most recent transaction stop time
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_tup_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of index entries returned by scans on this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_tup_fetch</structfield> <type>bigint</type>
      </para>
      <para>
       Number of live table rows fetched by simple index scans using this
       index
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   Indexes can be used by simple index scans, <quote>bitmap</quote> index scans,
   and the optimizer.  In a bitmap scan
   the output of several indexes can be combined via AND or OR rules,
   so it is difficult to associate individual heap row fetches
   with specific indexes when a bitmap scan is used.  Therefore, a bitmap
   scan increments the
   <structname>pg_stat_all_indexes</structname>.<structfield>idx_tup_read</structfield>
   count(s) for the index(es) it uses, and it increments the
   <structname>pg_stat_all_tables</structname>.<structfield>idx_tup_fetch</structfield>
   count for the table, but it does not affect
   <structname>pg_stat_all_indexes</structname>.<structfield>idx_tup_fetch</structfield>.
   The optimizer also accesses indexes to check for supplied constants
   whose values are outside the recorded range of the optimizer statistics
   because the optimizer statistics might be stale.
  </para>

  <note>
   <para>
    The <structfield>idx_tup_read</structfield> and <structfield>idx_tup_fetch</structfield> counts
    can be different even without any use of bitmap scans,
    because <structfield>idx_tup_read</structfield> counts
    index entries retrieved from the index while <structfield>idx_tup_fetch</structfield>
    counts live rows fetched from the table.  The latter will be less if any
    dead or not-yet-committed rows are fetched using the index, or if any
    heap fetches are avoided by means of an index-only scan.
   </para>
  </note>

  <note>
   <para>
    Queries that use certain <acronym>SQL</acronym> constructs to search for
    rows matching any value out of a list or array of multiple scalar values
    (see <xref linkend="functions-comparisons"/>) perform multiple
    <quote>primitive</quote> index scans (up to one primitive scan per scalar
    value) during query execution.  Each internal primitive index scan
    increments <structname>pg_stat_all_indexes</structname>.<structfield>idx_scan</structfield>,
    so it's possible for the count of index scans to significantly exceed the
    total number of index scan executor node executions.
   </para>
  </note>

 </sect2>

 <sect2 id="monitoring-pg-statio-all-tables-view">
  <title><structname>pg_statio_all_tables</structname></title>

  <indexterm>
   <primary>pg_statio_all_tables</primary>
  </indexterm>

  <para>
   The <structname>pg_statio_all_tables</structname> view will contain
   one row for each table in the current database (including TOAST
   tables), showing statistics about I/O on that specific table. The
   <structname>pg_statio_user_tables</structname> and
   <structname>pg_statio_sys_tables</structname> views
   contain the same information,
   but filtered to only show user and system tables respectively.
  </para>

  <table id="pg-statio-all-tables-view" xreflabel="pg_statio_all_tables">
   <title><structname>pg_statio_all_tables</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of a table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
      </para>
      <para>
       Name of the schema that this table is in
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relname</structfield> <type>name</type>
      </para>
      <para>
       Name of this table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_blks_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks read from this table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_blks_hit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffer hits in this table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_blks_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks read from all indexes on this table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_blks_hit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffer hits in all indexes on this table
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>toast_blks_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks read from this table's TOAST table (if any)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>toast_blks_hit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffer hits in this table's TOAST table (if any)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tidx_blks_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks read from this table's TOAST table indexes (if any)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tidx_blks_hit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffer hits in this table's TOAST table indexes (if any)
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-statio-all-indexes-view">
  <title><structname>pg_statio_all_indexes</structname></title>

  <indexterm>
   <primary>pg_statio_all_indexes</primary>
  </indexterm>

  <para>
   The <structname>pg_statio_all_indexes</structname> view will contain
   one row for each index in the current database,
   showing statistics about I/O on that specific index. The
   <structname>pg_statio_user_indexes</structname> and
   <structname>pg_statio_sys_indexes</structname> views
   contain the same information,
   but filtered to only show user and system indexes respectively.
  </para>

  <table id="pg-statio-all-indexes-view" xreflabel="pg_statio_all_indexes">
   <title><structname>pg_statio_all_indexes</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the table for this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>indexrelid</structfield> <type>oid</type>
      </para>
      <para>
       OID of this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
      </para>
      <para>
       Name of the schema this index is in
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relname</structfield> <type>name</type>
      </para>
      <para>
       Name of the table for this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>indexrelname</structfield> <type>name</type>
      </para>
      <para>
       Name of this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_blks_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks read from this index
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>idx_blks_hit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffer hits in this index
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-statio-all-sequences-view">
  <title><structname>pg_statio_all_sequences</structname></title>

  <indexterm>
   <primary>pg_statio_all_sequences</primary>
  </indexterm>

  <para>
   The <structname>pg_statio_all_sequences</structname> view will contain
   one row for each sequence in the current database,
   showing statistics about I/O on that specific sequence.
  </para>

  <table id="pg-statio-all-sequences-view" xreflabel="pg_statio_all_sequences">
   <title><structname>pg_statio_all_sequences</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of a sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
      </para>
      <para>
       Name of the schema this sequence is in
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relname</structfield> <type>name</type>
      </para>
      <para>
       Name of this sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks read from this sequence
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_hit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of buffer hits in this sequence
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-user-functions-view">
  <title><structname>pg_stat_user_functions</structname></title>

  <indexterm>
   <primary>pg_stat_user_functions</primary>
  </indexterm>

  <para>
   The <structname>pg_stat_user_functions</structname> view will contain
   one row for each tracked function, showing statistics about executions of
   that function.  The <xref linkend="guc-track-functions"/> parameter
   controls exactly which functions are tracked.
  </para>

  <table id="pg-stat-user-functions-view" xreflabel="pg_stat_user_functions">
   <title><structname>pg_stat_user_functions</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>funcid</structfield> <type>oid</type>
      </para>
      <para>
       OID of a function
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>schemaname</structfield> <type>name</type>
      </para>
      <para>
       Name of the schema this function is in
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>funcname</structfield> <type>name</type>
      </para>
      <para>
       Name of this function
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>calls</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times this function has been called
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>total_time</structfield> <type>double precision</type>
      </para>
      <para>
       Total time spent in this function and all other functions
       called by it, in milliseconds
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>self_time</structfield> <type>double precision</type>
      </para>
      <para>
       Total time spent in this function itself, not including
       other functions called by it, in milliseconds
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-pg-stat-slru-view">
  <title><structname>pg_stat_slru</structname></title>

  <indexterm>
   <primary>SLRU</primary>
  </indexterm>

  <indexterm>
   <primary>pg_stat_slru</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> accesses certain on-disk information
   via <literal>SLRU</literal> (<firstterm>simple least-recently-used</firstterm>)
   caches.
   The <structname>pg_stat_slru</structname> view will contain
   one row for each tracked SLRU cache, showing statistics about access
   to cached pages.
  </para>

  <para>
   For each <literal>SLRU</literal> cache that's part of the core server,
   there is a configuration parameter that controls its size, with the suffix
   <literal>_buffers</literal> appended.
  </para>

  <table id="pg-stat-slru-view" xreflabel="pg_stat_slru">
   <title><structname>pg_stat_slru</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>name</structfield> <type>text</type>
      </para>
      <para>
       Name of the SLRU
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_zeroed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of blocks zeroed during initializations
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_hit</structfield> <type>bigint</type>
      </para>
      <para>
       Number of times disk blocks were found already in the SLRU,
       so that a read was not necessary (this only includes hits in the
       SLRU, not the operating system's file system cache)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_read</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks read for this SLRU
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_written</structfield> <type>bigint</type>
      </para>
      <para>
       Number of disk blocks written for this SLRU
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blks_exists</structfield> <type>bigint</type>
      </para>
      <para>
       Number of blocks checked for existence for this SLRU
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>flushes</structfield> <type>bigint</type>
      </para>
      <para>
       Number of flushes of dirty data for this SLRU
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>truncates</structfield> <type>bigint</type>
      </para>
      <para>
       Number of truncates for this SLRU
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
      </para>
      <para>
       Time at which these statistics were last reset
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="monitoring-stats-functions">
  <title>Statistics Functions</title>

  <para>
   Other ways of looking at the statistics can be set up by writing
   queries that use the same underlying statistics access functions used by
   the standard views shown above.  For details such as the functions' names,
   consult the definitions of the standard views.  (For example, in
   <application>psql</application> you could issue <literal>\d+ pg_stat_activity</literal>.)
   The access functions for per-database statistics take a database OID as an
   argument to identify which database to report on.
   The per-table and per-index functions take a table or index OID.
   The functions for per-function statistics take a function OID.
   Note that only tables, indexes, and functions in the current database
   can be seen with these functions.
  </para>

  <para>
   Additional functions related to the cumulative statistics system are listed
   in <xref linkend="monitoring-stats-funcs-table"/>.
  </para>

   <table id="monitoring-stats-funcs-table">
    <title>Additional Statistics Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <!-- See also the entry for this in func.sgml -->
       <entry role="func_table_entry"><para role="func_signature">
        <function>pg_backend_pid</function> ()
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the process ID of the server process attached to the current
        session.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_activity</primary>
        </indexterm>
        <function>pg_stat_get_activity</function> ( <type>integer</type> )
        <returnvalue>setof record</returnvalue>
       </para>
       <para>
        Returns a record of information about the backend with the specified
        process ID, or one record for each active backend in the system
        if <literal>NULL</literal> is specified.  The fields returned are a
        subset of those in the <structname>pg_stat_activity</structname> view.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_snapshot_timestamp</primary>
        </indexterm>
        <function>pg_stat_get_snapshot_timestamp</function> ()
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the timestamp of the current statistics snapshot, or NULL if
        no statistics snapshot has been taken. A snapshot is taken the first
        time cumulative statistics are accessed in a transaction if
        <varname>stats_fetch_consistency</varname> is set to
        <literal>snapshot</literal>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_xact_blocks_fetched</primary>
        </indexterm>
        <function>pg_stat_get_xact_blocks_fetched</function> ( <type>oid</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Returns the number of block read requests for table or index, in the
        current transaction. This number minus
        <function>pg_stat_get_xact_blocks_hit</function> gives the number of
        kernel <function>read()</function> calls; the number of actual
        physical reads is usually lower due to kernel-level buffering.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_xact_blocks_hit</primary>
        </indexterm>
        <function>pg_stat_get_xact_blocks_hit</function> ( <type>oid</type> )
        <returnvalue>bigint</returnvalue>
       </para>
       <para>
        Returns the number of block read requests for table or index, in the
        current transaction, found in cache (not triggering kernel
        <function>read()</function> calls).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_clear_snapshot</primary>
        </indexterm>
        <function>pg_stat_clear_snapshot</function> ()
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Discards the current statistics snapshot or cached information.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_reset</primary>
        </indexterm>
        <function>pg_stat_reset</function> ()
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Resets all statistics counters for the current database to zero.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_reset_shared</primary>
        </indexterm>
        <function>pg_stat_reset_shared</function> ( [ <parameter>target</parameter> <type>text</type> <literal>DEFAULT</literal> <literal>NULL</literal> ] )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Resets some cluster-wide statistics counters to zero, depending on the
        argument. <parameter>target</parameter> can be:
       <itemizedlist>
        <listitem>
         <para>
          <literal>archiver</literal>: Reset all the counters shown in the
          <structname>pg_stat_archiver</structname> view.
         </para>
        </listitem>
        <listitem>
         <para>
           <literal>bgwriter</literal>: Reset all the counters shown in the
           <structname>pg_stat_bgwriter</structname> view.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>checkpointer</literal>: Reset all the counters shown in the
          <structname>pg_stat_checkpointer</structname> view.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>io</literal>: Reset all the counters shown in the
          <structname>pg_stat_io</structname> view.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>recovery_prefetch</literal>: Reset all the counters shown in
          the <structname>pg_stat_recovery_prefetch</structname> view.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>slru</literal>: Reset all the counters shown in the
          <structname>pg_stat_slru</structname> view.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>wal</literal>: Reset all the counters shown in the
          <structname>pg_stat_wal</structname> view.
         </para>
        </listitem>
        <listitem>
         <para>
          <literal>NULL</literal> or not specified: All the counters from the
          views listed above are reset.
         </para>
        </listitem>
       </itemizedlist>
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_reset_single_table_counters</primary>
        </indexterm>
        <function>pg_stat_reset_single_table_counters</function> ( <type>oid</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Resets statistics for a single table or index in the current database
        or shared across all databases in the cluster to zero.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_reset_single_function_counters</primary>
        </indexterm>
        <function>pg_stat_reset_single_function_counters</function> ( <type>oid</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Resets statistics for a single function in the current database to
        zero.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_reset_slru</primary>
        </indexterm>
        <function>pg_stat_reset_slru</function> ( [ <parameter>target</parameter> <type>text</type> <literal>DEFAULT</literal> <literal>NULL</literal> ] )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Resets statistics to zero for a single SLRU cache, or for all SLRUs in
        the cluster. If <parameter>target</parameter> is
        <literal>NULL</literal> or is not specified, all the counters shown in
        the <structname>pg_stat_slru</structname> view for all SLRU caches are
        reset. The argument can be one of
        <literal>commit_timestamp</literal>,
        <literal>multixact_member</literal>,
        <literal>multixact_offset</literal>,
        <literal>notify</literal>,
        <literal>serializable</literal>,
        <literal>subtransaction</literal>, or
        <literal>transaction</literal>
        to reset the counters for only that entry.
        If the argument is <literal>other</literal> (or indeed, any
        unrecognized name), then the counters for all other SLRU caches, such
        as extension-defined caches, are reset.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
          <primary>pg_stat_reset_replication_slot</primary>
        </indexterm>
        <function>pg_stat_reset_replication_slot</function> ( <type>text</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Resets statistics of the replication slot defined by the argument. If
        the argument is <literal>NULL</literal>, resets statistics for all
        the replication slots.
       </para>
       <para>
         This function is restricted to superusers by default, but other users
         can be granted EXECUTE to run the function.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
          <primary>pg_stat_reset_subscription_stats</primary>
        </indexterm>
        <function>pg_stat_reset_subscription_stats</function> ( <type>oid</type> )
        <returnvalue>void</returnvalue>
       </para>
       <para>
        Resets statistics for a single subscription shown in the
        <structname>pg_stat_subscription_stats</structname> view to zero. If
        the argument is <literal>NULL</literal>, reset statistics for all
        subscriptions.
       </para>
       <para>
        This function is restricted to superusers by default, but other users
        can be granted EXECUTE to run the function.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <warning>
   <para>
    Using <function>pg_stat_reset()</function> also resets counters that
    autovacuum uses to determine when to trigger a vacuum or an analyze.
    Resetting these counters can cause autovacuum to not perform necessary
    work, which can cause problems such as table bloat or out-dated
    table statistics.  A database-wide <command>ANALYZE</command> is
    recommended after the statistics have been reset.
   </para>
  </warning>

  <para>
   <function>pg_stat_get_activity</function>, the underlying function of
   the <structname>pg_stat_activity</structname> view, returns a set of records
   containing all the available information about each backend process.
   Sometimes it may be more convenient to obtain just a subset of this
   information.  In such cases, another set of per-backend statistics
   access functions can be used; these are shown in <xref
   linkend="monitoring-stats-backend-funcs-table"/>.
   These access functions use the session's backend ID number, which is a
   small integer (>= 0) that is distinct from the backend ID of any
   concurrent session, although a session's ID can be recycled as soon as
   it exits.  The backend ID is used, among other things, to identify the
   session's temporary schema if it has one.
   The function <function>pg_stat_get_backend_idset</function> provides a
   convenient way to list all the active backends' ID numbers for
   invoking these functions.  For example, to show the <acronym>PID</acronym>s and
   current queries of all backends:

<programlisting>
SELECT pg_stat_get_backend_pid(backendid) AS pid,
       pg_stat_get_backend_activity(backendid) AS query
FROM pg_stat_get_backend_idset() AS backendid;
</programlisting>
  </para>

   <table id="monitoring-stats-backend-funcs-table">
    <title>Per-Backend Statistics Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_activity</primary>
        </indexterm>
        <function>pg_stat_get_backend_activity</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the text of this backend's most recent query.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_activity_start</primary>
        </indexterm>
        <function>pg_stat_get_backend_activity_start</function> ( <type>integer</type> )
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the time when the backend's most recent query was started.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_client_addr</primary>
        </indexterm>
        <function>pg_stat_get_backend_client_addr</function> ( <type>integer</type> )
        <returnvalue>inet</returnvalue>
       </para>
       <para>
        Returns the IP address of the client connected to this backend.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_client_port</primary>
        </indexterm>
        <function>pg_stat_get_backend_client_port</function> ( <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the TCP port number that the client is using for communication.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_dbid</primary>
        </indexterm>
        <function>pg_stat_get_backend_dbid</function> ( <type>integer</type> )
        <returnvalue>oid</returnvalue>
       </para>
       <para>
        Returns the OID of the database this backend is connected to.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_idset</primary>
        </indexterm>
        <function>pg_stat_get_backend_idset</function> ()
        <returnvalue>setof integer</returnvalue>
       </para>
       <para>
        Returns the set of currently active backend ID numbers.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_pid</primary>
        </indexterm>
        <function>pg_stat_get_backend_pid</function> ( <type>integer</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the process ID of this backend.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_start</primary>
        </indexterm>
        <function>pg_stat_get_backend_start</function> ( <type>integer</type> )
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the time when this process was started.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_subxact</primary>
        </indexterm>
        <function>pg_stat_get_backend_subxact</function> ( <type>integer</type> )
        <returnvalue>record</returnvalue>
       </para>
       <para>
        Returns a record of information about the subtransactions of the
        backend with the specified ID.
        The fields returned are <parameter>subxact_count</parameter>, which
        is the number of subtransactions in the backend's subtransaction cache,
        and <parameter>subxact_overflow</parameter>, which indicates whether
        the backend's subtransaction cache is overflowed or not.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_userid</primary>
        </indexterm>
        <function>pg_stat_get_backend_userid</function> ( <type>integer</type> )
        <returnvalue>oid</returnvalue>
       </para>
       <para>
        Returns the OID of the user logged into this backend.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_wait_event</primary>
        </indexterm>
        <function>pg_stat_get_backend_wait_event</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the wait event name if this backend is currently waiting,
        otherwise NULL. See <xref linkend="wait-event-activity-table"/> through
        <xref linkend="wait-event-timeout-table"/>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_wait_event_type</primary>
        </indexterm>
        <function>pg_stat_get_backend_wait_event_type</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the wait event type name if this backend is currently waiting,
        otherwise NULL.  See <xref linkend="wait-event-table"/> for details.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_stat_get_backend_xact_start</primary>
        </indexterm>
        <function>pg_stat_get_backend_xact_start</function> ( <type>integer</type> )
        <returnvalue>timestamp with time zone</returnvalue>
       </para>
       <para>
        Returns the time when the backend's current transaction was started.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

 </sect2>
 </sect1>

 <sect1 id="monitoring-locks">
  <title>Viewing Locks</title>

  <indexterm zone="monitoring-locks">
   <primary>lock</primary>
   <secondary>monitoring</secondary>
  </indexterm>

  <para>
   Another useful tool for monitoring database activity is the
   <structname>pg_locks</structname> system table.  It allows the
   database administrator to view information about the outstanding
   locks in the lock manager. For example, this capability can be used
   to:

   <itemizedlist>
    <listitem>
     <para>
      View all the locks currently outstanding, all the locks on
      relations in a particular database, all the locks on a
      particular relation, or all the locks held by a particular
      <productname>PostgreSQL</productname> session.
     </para>
    </listitem>

    <listitem>
     <para>
      Determine the relation in the current database with the most
      ungranted locks (which might be a source of contention among
      database clients).
     </para>
    </listitem>

    <listitem>
     <para>
      Determine the effect of lock contention on overall database
      performance, as well as the extent to which contention varies
      with overall database traffic.
     </para>
    </listitem>
   </itemizedlist>

   Details of the <structname>pg_locks</structname> view appear in
   <xref linkend="view-pg-locks"/>.
   For more information on locking and managing concurrency with
   <productname>PostgreSQL</productname>, refer to <xref linkend="mvcc"/>.
  </para>
 </sect1>

 <sect1 id="progress-reporting">
  <title>Progress Reporting</title>

  <para>
   <productname>PostgreSQL</productname> has the ability to report the progress of
   certain commands during command execution.  Currently, the only commands
   which support progress reporting are <command>ANALYZE</command>,
   <command>CLUSTER</command>,
   <command>CREATE INDEX</command>, <command>VACUUM</command>,
   <command>COPY</command>,
   and <xref linkend="protocol-replication-base-backup"/> (i.e., replication
   command that <xref linkend="app-pgbasebackup"/> issues to take
   a base backup).
   This may be expanded in the future.
  </para>

 <sect2 id="analyze-progress-reporting">
  <title>ANALYZE Progress Reporting</title>

  <indexterm>
   <primary>pg_stat_progress_analyze</primary>
  </indexterm>

  <para>
   Whenever <command>ANALYZE</command> is running, the
   <structname>pg_stat_progress_analyze</structname> view will contain a
   row for each backend that is currently running that command.  The tables
   below describe the information that will be reported and provide
   information about how to interpret it.
  </para>

  <table id="pg-stat-progress-analyze-view" xreflabel="pg_stat_progress_analyze">
   <title><structname>pg_stat_progress_analyze</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of backend.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datname</structfield> <type>name</type>
      </para>
      <para>
       Name of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the table being analyzed.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>phase</structfield> <type>text</type>
      </para>
      <para>
       Current processing phase. See <xref linkend="analyze-phases"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sample_blks_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of heap blocks that will be sampled.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>sample_blks_scanned</structfield> <type>bigint</type>
      </para>
      <para>
       Number of heap blocks scanned.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>ext_stats_total</structfield> <type>bigint</type>
      </para>
      <para>
       Number of extended statistics.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>ext_stats_computed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of extended statistics computed. This counter only advances
       when the phase is <literal>computing extended statistics</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>child_tables_total</structfield> <type>bigint</type>
      </para>
      <para>
       Number of child tables.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>child_tables_done</structfield> <type>bigint</type>
      </para>
      <para>
       Number of child tables scanned. This counter only advances when the
       phase is <literal>acquiring inherited sample rows</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>current_child_table_relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the child table currently being scanned. This field is
       only valid when the phase is
       <literal>acquiring inherited sample rows</literal>.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="analyze-phases">
   <title>ANALYZE Phases</title>
   <tgroup cols="2">
    <colspec colname="col1" colwidth="1*"/>
    <colspec colname="col2" colwidth="2*"/>
    <thead>
     <row>
      <entry>Phase</entry>
      <entry>Description</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><literal>initializing</literal></entry>
      <entry>
       The command is preparing to begin scanning the heap.  This phase is
       expected to be very brief.
      </entry>
     </row>
     <row>
      <entry><literal>acquiring sample rows</literal></entry>
      <entry>
       The command is currently scanning the table given by
       <structfield>relid</structfield> to obtain sample rows.
      </entry>
     </row>
     <row>
      <entry><literal>acquiring inherited sample rows</literal></entry>
      <entry>
       The command is currently scanning child tables to obtain sample rows.
       Columns <structfield>child_tables_total</structfield>,
       <structfield>child_tables_done</structfield>, and
       <structfield>current_child_table_relid</structfield> contain the
       progress information for this phase.
      </entry>
     </row>
     <row>
      <entry><literal>computing statistics</literal></entry>
      <entry>
       The command is computing statistics from the sample rows obtained
       during the table scan.
      </entry>
     </row>
     <row>
      <entry><literal>computing extended statistics</literal></entry>
      <entry>
       The command is computing extended statistics from the sample rows
       obtained during the table scan.
      </entry>
     </row>
     <row>
      <entry><literal>finalizing analyze</literal></entry>
      <entry>
       The command is updating <structname>pg_class</structname>. When this
       phase is completed, <command>ANALYZE</command> will end.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <note>
   <para>
    Note that when <command>ANALYZE</command> is run on a partitioned table,
    all of its partitions are also recursively analyzed.
    In that case, <command>ANALYZE</command>
    progress is reported first for the parent table, whereby its inheritance
    statistics are collected, followed by that for each partition.
   </para>
  </note>
 </sect2>

 <sect2 id="cluster-progress-reporting">
  <title>CLUSTER Progress Reporting</title>

  <indexterm>
   <primary>pg_stat_progress_cluster</primary>
  </indexterm>

  <para>
   Whenever <command>CLUSTER</command> or <command>VACUUM FULL</command> is
   running, the <structname>pg_stat_progress_cluster</structname> view will
   contain a row for each backend that is currently running either command.
   The tables below describe the information that will be reported and
   provide information about how to interpret it.
  </para>

  <table id="pg-stat-progress-cluster-view" xreflabel="pg_stat_progress_cluster">
   <title><structname>pg_stat_progress_cluster</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of backend.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datname</structfield> <type>name</type>
      </para>
      <para>
       Name of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the table being clustered.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>command</structfield> <type>text</type>
      </para>
      <para>
       The command that is running. Either <literal>CLUSTER</literal> or <literal>VACUUM FULL</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>phase</structfield> <type>text</type>
      </para>
      <para>
       Current processing phase. See <xref linkend="cluster-phases"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>cluster_index_relid</structfield> <type>oid</type>
      </para>
      <para>
       If the table is being scanned using an index, this is the OID of the
       index being used; otherwise, it is zero.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_tuples_scanned</structfield> <type>bigint</type>
      </para>
      <para>
       Number of heap tuples scanned.
       This counter only advances when the phase is
       <literal>seq scanning heap</literal>,
       <literal>index scanning heap</literal>
       or <literal>writing new heap</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_tuples_written</structfield> <type>bigint</type>
      </para>
      <para>
       Number of heap tuples written.
       This counter only advances when the phase is
       <literal>seq scanning heap</literal>,
       <literal>index scanning heap</literal>
       or <literal>writing new heap</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_blks_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of heap blocks in the table.  This number is reported
       as of the beginning of <literal>seq scanning heap</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_blks_scanned</structfield> <type>bigint</type>
      </para>
      <para>
       Number of heap blocks scanned.  This counter only advances when the
       phase is <literal>seq scanning heap</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>index_rebuild_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of indexes rebuilt.  This counter only advances when the phase
       is <literal>rebuilding index</literal>.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="cluster-phases">
   <title>CLUSTER and VACUUM FULL Phases</title>
   <tgroup cols="2">
    <colspec colname="col1" colwidth="1*"/>
    <colspec colname="col2" colwidth="2*"/>
    <thead>
    <row>
      <entry>Phase</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><literal>initializing</literal></entry>
     <entry>
       The command is preparing to begin scanning the heap.  This phase is
       expected to be very brief.
     </entry>
    </row>
    <row>
     <entry><literal>seq scanning heap</literal></entry>
     <entry>
       The command is currently scanning the table using a sequential scan.
     </entry>
    </row>
    <row>
     <entry><literal>index scanning heap</literal></entry>
     <entry>
       <command>CLUSTER</command> is currently scanning the table using an index scan.
     </entry>
    </row>
    <row>
     <entry><literal>sorting tuples</literal></entry>
     <entry>
       <command>CLUSTER</command> is currently sorting tuples.
     </entry>
    </row>
    <row>
     <entry><literal>writing new heap</literal></entry>
     <entry>
       <command>CLUSTER</command> is currently writing the new heap.
     </entry>
    </row>
    <row>
     <entry><literal>swapping relation files</literal></entry>
     <entry>
       The command is currently swapping newly-built files into place.
     </entry>
    </row>
    <row>
     <entry><literal>rebuilding index</literal></entry>
     <entry>
       The command is currently rebuilding an index.
     </entry>
    </row>
    <row>
     <entry><literal>performing final cleanup</literal></entry>
     <entry>
       The command is performing final cleanup.  When this phase is
       completed, <command>CLUSTER</command>
       or <command>VACUUM FULL</command> will end.
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>
 </sect2>

 <sect2 id="copy-progress-reporting">
  <title>COPY Progress Reporting</title>

  <indexterm>
   <primary>pg_stat_progress_copy</primary>
  </indexterm>

  <para>
   Whenever <command>COPY</command> is running, the
   <structname>pg_stat_progress_copy</structname> view will contain one row
   for each backend that is currently running a <command>COPY</command> command.
   The table below describes the information that will be reported and provides
   information about how to interpret it.
  </para>

  <table id="pg-stat-progress-copy-view" xreflabel="pg_stat_progress_copy">
   <title><structname>pg_stat_progress_copy</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of backend.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datname</structfield> <type>name</type>
      </para>
      <para>
       Name of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the table on which the <command>COPY</command> command is
       executed. It is set to <literal>0</literal> if copying from a
       <command>SELECT</command> query.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>command</structfield> <type>text</type>
      </para>
      <para>
       The command that is running: <literal>COPY FROM</literal>, or
       <literal>COPY TO</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>type</structfield> <type>text</type>
      </para>
      <para>
       The I/O type that the data is read from or written to:
       <literal>FILE</literal>, <literal>PROGRAM</literal>,
       <literal>PIPE</literal> (for <command>COPY FROM STDIN</command> and
       <command>COPY TO STDOUT</command>), or <literal>CALLBACK</literal>
       (used for example during the initial table synchronization in
       logical replication).
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>bytes_processed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of bytes already processed by <command>COPY</command> command.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>bytes_total</structfield> <type>bigint</type>
      </para>
      <para>
       Size of source file for <command>COPY FROM</command> command in bytes.
       It is set to <literal>0</literal> if not available.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tuples_processed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of tuples already processed by <command>COPY</command> command.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tuples_excluded</structfield> <type>bigint</type>
      </para>
      <para>
       Number of tuples not processed because they were excluded by the
       <command>WHERE</command> clause of the <command>COPY</command> command.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tuples_skipped</structfield> <type>bigint</type>
      </para>
      <para>
       Number of tuples skipped because they contain malformed data.
       This counter only advances when a value other than
       <literal>stop</literal> is specified to the <literal>ON_ERROR</literal>
       option.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>
 </sect2>

 <sect2 id="create-index-progress-reporting">
  <title>CREATE INDEX Progress Reporting</title>

  <indexterm>
   <primary>pg_stat_progress_create_index</primary>
  </indexterm>

  <para>
   Whenever <command>CREATE INDEX</command> or <command>REINDEX</command> is running, the
   <structname>pg_stat_progress_create_index</structname> view will contain
   one row for each backend that is currently creating indexes.  The tables
   below describe the information that will be reported and provide information
   about how to interpret it.
  </para>

  <table id="pg-stat-progress-create-index-view" xreflabel="pg_stat_progress_create_index">
   <title><structname>pg_stat_progress_create_index</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of the backend creating indexes.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datname</structfield> <type>name</type>
      </para>
      <para>
       Name of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the table on which the index is being created.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>index_relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the index being created or reindexed.  During a
       non-concurrent <command>CREATE INDEX</command>, this is 0.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>command</structfield> <type>text</type>
      </para>
      <para>
       Specific command type: <literal>CREATE INDEX</literal>,
       <literal>CREATE INDEX CONCURRENTLY</literal>,
       <literal>REINDEX</literal>, or <literal>REINDEX CONCURRENTLY</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>phase</structfield> <type>text</type>
      </para>
      <para>
       Current processing phase of index creation.  See <xref linkend="create-index-phases"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>lockers_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of lockers to wait for, when applicable.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>lockers_done</structfield> <type>bigint</type>
      </para>
      <para>
       Number of lockers already waited for.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>current_locker_pid</structfield> <type>bigint</type>
      </para>
      <para>
       Process ID of the locker currently being waited for.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blocks_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of blocks to be processed in the current phase.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>blocks_done</structfield> <type>bigint</type>
      </para>
      <para>
       Number of blocks already processed in the current phase.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tuples_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of tuples to be processed in the current phase.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tuples_done</structfield> <type>bigint</type>
      </para>
      <para>
       Number of tuples already processed in the current phase.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>partitions_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of partitions on which the index is to be created
       or attached, including both direct and indirect partitions.
       <literal>0</literal> during a <literal>REINDEX</literal>, or when
       the index is not partitioned.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>partitions_done</structfield> <type>bigint</type>
      </para>
      <para>
       Number of partitions on which the index has already been created
       or attached, including both direct and indirect partitions.
       <literal>0</literal> during a <literal>REINDEX</literal>, or when
       the index is not partitioned.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="create-index-phases">
   <title>CREATE INDEX Phases</title>
   <tgroup cols="2">
    <colspec colname="col1" colwidth="1*"/>
    <colspec colname="col2" colwidth="2*"/>
    <thead>
     <row>
      <entry>Phase</entry>
      <entry>Description</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><literal>initializing</literal></entry>
      <entry>
       <command>CREATE INDEX</command> or <command>REINDEX</command> is preparing to create the index.  This
       phase is expected to be very brief.
      </entry>
     </row>
     <row>
      <entry><literal>waiting for writers before build</literal></entry>
      <entry>
       <command>CREATE INDEX CONCURRENTLY</command> or <command>REINDEX CONCURRENTLY</command> is waiting for transactions
       with write locks that can potentially see the table to finish.
       This phase is skipped when not in concurrent mode.
       Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
       and <structname>current_locker_pid</structname> contain the progress
       information for this phase.
      </entry>
     </row>
     <row>
      <entry><literal>building index</literal></entry>
      <entry>
       The index is being built by the access method-specific code.  In this phase,
       access methods that support progress reporting fill in their own progress data,
       and the subphase is indicated in this column.  Typically,
       <structname>blocks_total</structname> and <structname>blocks_done</structname>
       will contain progress data, as well as potentially
       <structname>tuples_total</structname> and <structname>tuples_done</structname>.
      </entry>
     </row>
     <row>
      <entry><literal>waiting for writers before validation</literal></entry>
      <entry>
       <command>CREATE INDEX CONCURRENTLY</command> or <command>REINDEX CONCURRENTLY</command> is waiting for transactions
       with write locks that can potentially write into the table to finish.
       This phase is skipped when not in concurrent mode.
       Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
       and <structname>current_locker_pid</structname> contain the progress
       information for this phase.
      </entry>
     </row>
     <row>
      <entry><literal>index validation: scanning index</literal></entry>
      <entry>
       <command>CREATE INDEX CONCURRENTLY</command> is scanning the index searching
       for tuples that need to be validated.
       This phase is skipped when not in concurrent mode.
       Columns <structname>blocks_total</structname> (set to the total size of the index)
       and <structname>blocks_done</structname> contain the progress information for this phase.
      </entry>
     </row>
     <row>
      <entry><literal>index validation: sorting tuples</literal></entry>
      <entry>
       <command>CREATE INDEX CONCURRENTLY</command> is sorting the output of the
       index scanning phase.
      </entry>
     </row>
     <row>
      <entry><literal>index validation: scanning table</literal></entry>
      <entry>
       <command>CREATE INDEX CONCURRENTLY</command> is scanning the table
       to validate the index tuples collected in the previous two phases.
       This phase is skipped when not in concurrent mode.
       Columns <structname>blocks_total</structname> (set to the total size of the table)
       and <structname>blocks_done</structname> contain the progress information for this phase.
      </entry>
     </row>
     <row>
      <entry><literal>waiting for old snapshots</literal></entry>
      <entry>
       <command>CREATE INDEX CONCURRENTLY</command> or <command>REINDEX CONCURRENTLY</command> is waiting for transactions
       that can potentially see the table to release their snapshots.  This
       phase is skipped when not in concurrent mode.
       Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
       and <structname>current_locker_pid</structname> contain the progress
       information for this phase.
      </entry>
     </row>
     <row>
      <entry><literal>waiting for readers before marking dead</literal></entry>
      <entry>
       <command>REINDEX CONCURRENTLY</command> is waiting for transactions
       with read locks on the table to finish, before marking the old index dead.
       This phase is skipped when not in concurrent mode.
       Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
       and <structname>current_locker_pid</structname> contain the progress
       information for this phase.
      </entry>
     </row>
     <row>
      <entry><literal>waiting for readers before dropping</literal></entry>
      <entry>
       <command>REINDEX CONCURRENTLY</command> is waiting for transactions
       with read locks on the table to finish, before dropping the old index.
       This phase is skipped when not in concurrent mode.
       Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
       and <structname>current_locker_pid</structname> contain the progress
       information for this phase.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 <sect2 id="vacuum-progress-reporting">
  <title>VACUUM Progress Reporting</title>

  <indexterm>
   <primary>pg_stat_progress_vacuum</primary>
  </indexterm>

  <para>
   Whenever <command>VACUUM</command> is running, the
   <structname>pg_stat_progress_vacuum</structname> view will contain
   one row for each backend (including autovacuum worker processes) that is
   currently vacuuming.  The tables below describe the information
   that will be reported and provide information about how to interpret it.
   Progress for <command>VACUUM FULL</command> commands is reported via
   <structname>pg_stat_progress_cluster</structname>
   because both <command>VACUUM FULL</command> and <command>CLUSTER</command>
   rewrite the table, while regular <command>VACUUM</command> only modifies it
   in place. See <xref linkend="cluster-progress-reporting"/>.
  </para>

  <table id="pg-stat-progress-vacuum-view" xreflabel="pg_stat_progress_vacuum">
   <title><structname>pg_stat_progress_vacuum</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of backend.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>datname</structfield> <type>name</type>
      </para>
      <para>
       Name of the database to which this backend is connected.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relid</structfield> <type>oid</type>
      </para>
      <para>
       OID of the table being vacuumed.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>phase</structfield> <type>text</type>
      </para>
      <para>
       Current processing phase of vacuum.  See <xref linkend="vacuum-phases"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_blks_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of heap blocks in the table.  This number is reported
       as of the beginning of the scan; blocks added later will not be (and
       need not be) visited by this <command>VACUUM</command>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_blks_scanned</structfield> <type>bigint</type>
      </para>
      <para>
       Number of heap blocks scanned.  Because the
       <link linkend="storage-vm">visibility map</link> is used to optimize scans,
       some blocks will be skipped without inspection; skipped blocks are
       included in this total, so that this number will eventually become
       equal to <structfield>heap_blks_total</structfield> when the vacuum is complete.
       This counter only advances when the phase is <literal>scanning heap</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>heap_blks_vacuumed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of heap blocks vacuumed.  Unless the table has no indexes, this
       counter only advances when the phase is <literal>vacuuming heap</literal>.
       Blocks that contain no dead tuples are skipped, so the counter may
       sometimes skip forward in large increments.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>index_vacuum_count</structfield> <type>bigint</type>
      </para>
      <para>
       Number of completed index vacuum cycles.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>max_dead_tuple_bytes</structfield> <type>bigint</type>
      </para>
      <para>
       Amount of dead tuple data that we can store before needing to perform
       an index vacuum cycle, based on
       <xref linkend="guc-maintenance-work-mem"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>dead_tuple_bytes</structfield> <type>bigint</type>
      </para>
      <para>
       Amount of dead tuple data collected since the last index vacuum cycle.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
      <structfield>num_dead_item_ids</structfield> <type>bigint</type>
      </para>
      <para>
       Number of dead item identifiers collected since the last index vacuum cycle.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>indexes_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of indexes that will be vacuumed or cleaned up. This
       number is reported at the beginning of the
       <literal>vacuuming indexes</literal> phase or the
       <literal>cleaning up indexes</literal> phase.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>indexes_processed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of indexes processed. This counter only advances when the
       phase is <literal>vacuuming indexes</literal> or
       <literal>cleaning up indexes</literal>.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="vacuum-phases">
   <title>VACUUM Phases</title>
   <tgroup cols="2">
    <colspec colname="col1" colwidth="1*"/>
    <colspec colname="col2" colwidth="2*"/>
    <thead>
    <row>
      <entry>Phase</entry>
      <entry>Description</entry>
     </row>
    </thead>

   <tbody>
    <row>
     <entry><literal>initializing</literal></entry>
     <entry>
       <command>VACUUM</command> is preparing to begin scanning the heap.  This
       phase is expected to be very brief.
     </entry>
    </row>
    <row>
     <entry><literal>scanning heap</literal></entry>
     <entry>
       <command>VACUUM</command> is currently scanning the heap.  It will prune and
       defragment each page if required, and possibly perform freezing
       activity.  The <structfield>heap_blks_scanned</structfield> column can be used
       to monitor the progress of the scan.
     </entry>
    </row>
    <row>
     <entry><literal>vacuuming indexes</literal></entry>
     <entry>
       <command>VACUUM</command> is currently vacuuming the indexes.  If a table has
       any indexes, this will happen at least once per vacuum, after the heap
       has been completely scanned.  It may happen multiple times per vacuum
       if <xref linkend="guc-maintenance-work-mem"/> (or, in the case of autovacuum,
       <xref linkend="guc-autovacuum-work-mem"/> if set) is insufficient to store
       the number of dead tuples found.
     </entry>
    </row>
    <row>
     <entry><literal>vacuuming heap</literal></entry>
     <entry>
       <command>VACUUM</command> is currently vacuuming the heap.  Vacuuming the heap
       is distinct from scanning the heap, and occurs after each instance of
       vacuuming indexes.  If <structfield>heap_blks_scanned</structfield> is less than
       <structfield>heap_blks_total</structfield>, the system will return to scanning
       the heap after this phase is completed; otherwise, it will begin
       cleaning up indexes after this phase is completed.
     </entry>
    </row>
    <row>
     <entry><literal>cleaning up indexes</literal></entry>
     <entry>
       <command>VACUUM</command> is currently cleaning up indexes.  This occurs after
       the heap has been completely scanned and all vacuuming of the indexes
       and the heap has been completed.
     </entry>
    </row>
    <row>
     <entry><literal>truncating heap</literal></entry>
     <entry>
       <command>VACUUM</command> is currently truncating the heap so as to return
       empty pages at the end of the relation to the operating system.  This
       occurs after cleaning up indexes.
     </entry>
    </row>
    <row>
     <entry><literal>performing final cleanup</literal></entry>
     <entry>
       <command>VACUUM</command> is performing final cleanup.  During this phase,
       <command>VACUUM</command> will vacuum the free space map, update statistics
       in <literal>pg_class</literal>, and report statistics to the cumulative
       statistics system. When this phase is completed, <command>VACUUM</command> will end.
     </entry>
    </row>
   </tbody>
   </tgroup>
  </table>
 </sect2>

 <sect2 id="basebackup-progress-reporting">
  <title>Base Backup Progress Reporting</title>

  <indexterm>
   <primary>pg_stat_progress_basebackup</primary>
  </indexterm>

  <para>
   Whenever an application like <application>pg_basebackup</application>
   is taking a base backup, the
   <structname>pg_stat_progress_basebackup</structname>
   view will contain a row for each WAL sender process that is currently
   running the <command>BASE_BACKUP</command> replication command
   and streaming the backup. The tables below describe the information
   that will be reported and provide information about how to interpret it.
  </para>

  <table id="pg-stat-progress-basebackup-view" xreflabel="pg_stat_progress_basebackup">
   <title><structname>pg_stat_progress_basebackup</structname> View</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>pid</structfield> <type>integer</type>
      </para>
      <para>
       Process ID of a WAL sender process.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>phase</structfield> <type>text</type>
      </para>
      <para>
       Current processing phase. See <xref linkend="basebackup-phases"/>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>backup_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total amount of data that will be streamed. This is estimated and
       reported as of the beginning of
       <literal>streaming database files</literal> phase. Note that
       this is only an approximation since the database
       may change during <literal>streaming database files</literal> phase
       and WAL log may be included in the backup later. This is always
       the same value as <structfield>backup_streamed</structfield>
       once the amount of data streamed exceeds the estimated
       total size. If the estimation is disabled in
       <application>pg_basebackup</application>
       (i.e., <literal>--no-estimate-size</literal> option is specified),
       this is <literal>NULL</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>backup_streamed</structfield> <type>bigint</type>
      </para>
      <para>
       Amount of data streamed. This counter only advances
       when the phase is <literal>streaming database files</literal> or
       <literal>transferring wal files</literal>.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablespaces_total</structfield> <type>bigint</type>
      </para>
      <para>
       Total number of tablespaces that will be streamed.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>tablespaces_streamed</structfield> <type>bigint</type>
      </para>
      <para>
       Number of tablespaces streamed. This counter only
       advances when the phase is <literal>streaming database files</literal>.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="basebackup-phases">
   <title>Base Backup Phases</title>
   <tgroup cols="2">
    <colspec colname="col1" colwidth="1*"/>
    <colspec colname="col2" colwidth="2*"/>
    <thead>
     <row>
      <entry>Phase</entry>
      <entry>Description</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><literal>initializing</literal></entry>
      <entry>
       The WAL sender process is preparing to begin the backup.
       This phase is expected to be very brief.
      </entry>
     </row>
     <row>
      <entry><literal>waiting for checkpoint to finish</literal></entry>
      <entry>
       The WAL sender process is currently performing
       <function>pg_backup_start</function> to prepare to
       take a base backup, and waiting for the start-of-backup
       checkpoint to finish.
      </entry>
     </row>
     <row>
      <entry><literal>estimating backup size</literal></entry>
      <entry>
       The WAL sender process is currently estimating the total amount
       of database files that will be streamed as a base backup.
      </entry>
     </row>
     <row>
      <entry><literal>streaming database files</literal></entry>
      <entry>
       The WAL sender process is currently streaming database files
       as a base backup.
      </entry>
     </row>
     <row>
      <entry><literal>waiting for wal archiving to finish</literal></entry>
      <entry>
       The WAL sender process is currently performing
       <function>pg_backup_stop</function> to finish the backup,
       and waiting for all the WAL files required for the base backup
       to be successfully archived.
       If either <literal>--wal-method=none</literal> or
       <literal>--wal-method=stream</literal> is specified in
       <application>pg_basebackup</application>, the backup will end
       when this phase is completed.
      </entry>
     </row>
     <row>
      <entry><literal>transferring wal files</literal></entry>
      <entry>
       The WAL sender process is currently transferring all WAL logs
       generated during the backup. This phase occurs after
       <literal>waiting for wal archiving to finish</literal> phase if
       <literal>--wal-method=fetch</literal> is specified in
       <application>pg_basebackup</application>. The backup will end
       when this phase is completed.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

 </sect2>

 </sect1>

 <sect1 id="dynamic-trace">
  <title>Dynamic Tracing</title>

 <indexterm zone="dynamic-trace">
  <primary>DTrace</primary>
 </indexterm>

  <para>
   <productname>PostgreSQL</productname> provides facilities to support
   dynamic tracing of the database server. This allows an external
   utility to be called at specific points in the code and thereby trace
   execution.
  </para>

  <para>
   A number of probes or trace points are already inserted into the source
   code. These probes are intended to be used by database developers and
   administrators. By default the probes are not compiled into
   <productname>PostgreSQL</productname>; the user needs to explicitly tell
   the configure script to make the probes available.
  </para>

  <para>
   Currently, the
   <ulink url="https://en.wikipedia.org/wiki/DTrace">DTrace</ulink>
   utility is supported, which, at the time of this writing, is available
   on Solaris, macOS, FreeBSD, NetBSD, and Oracle Linux.  The
   <ulink url="https://sourceware.org/systemtap/">SystemTap</ulink> project
   for Linux provides a DTrace equivalent and can also be used.  Supporting other dynamic
   tracing utilities is theoretically possible by changing the definitions for
   the macros in <filename>src/include/utils/probes.h</filename>.
  </para>

  <sect2 id="compiling-for-trace">
   <title>Compiling for Dynamic Tracing</title>

  <para>
   By default, probes are not available, so you will need to
   explicitly tell the configure script to make the probes available
   in <productname>PostgreSQL</productname>. To include DTrace support
   specify <option>--enable-dtrace</option> to configure.  See <xref
   linkend="configure-options-devel"/> for further information.
  </para>
  </sect2>

  <sect2 id="trace-points">
   <title>Built-in Probes</title>

  <para>
   A number of standard probes are provided in the source code,
   as shown in <xref linkend="dtrace-probe-point-table"/>;
   <xref linkend="typedefs-table"/>
   shows the types used in the probes.  More probes can certainly be
   added to enhance <productname>PostgreSQL</productname>'s observability.
  </para>

 <table id="dtrace-probe-point-table">
  <title>Built-in DTrace Probes</title>
  <tgroup cols="3">
   <colspec colname="col1" colwidth="2*"/>
   <colspec colname="col2" colwidth="3*"/>
   <colspec colname="col3" colwidth="3*"/>
   <thead>
    <row>
     <entry>Name</entry>
     <entry>Parameters</entry>
     <entry>Description</entry>
    </row>
   </thead>

   <tbody>

    <row>
     <entry><literal>transaction-start</literal></entry>
     <entry><literal>(LocalTransactionId)</literal></entry>
     <entry>Probe that fires at the start of a new transaction.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry><literal>transaction-commit</literal></entry>
     <entry><literal>(LocalTransactionId)</literal></entry>
     <entry>Probe that fires when a transaction completes successfully.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry><literal>transaction-abort</literal></entry>
     <entry><literal>(LocalTransactionId)</literal></entry>
     <entry>Probe that fires when a transaction completes unsuccessfully.
      arg0 is the transaction ID.</entry>
    </row>
    <row>
     <entry><literal>query-start</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the processing of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-done</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the processing of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-parse-start</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the parsing of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-parse-done</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the parsing of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-rewrite-start</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the rewriting of a query is started.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-rewrite-done</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires when the rewriting of a query is complete.
      arg0 is the query string.</entry>
    </row>
    <row>
     <entry><literal>query-plan-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the planning of a query is started.</entry>
    </row>
    <row>
     <entry><literal>query-plan-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the planning of a query is complete.</entry>
    </row>
    <row>
     <entry><literal>query-execute-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the execution of a query is started.</entry>
    </row>
    <row>
     <entry><literal>query-execute-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the execution of a query is complete.</entry>
    </row>
    <row>
     <entry><literal>statement-status</literal></entry>
     <entry><literal>(const char *)</literal></entry>
     <entry>Probe that fires anytime the server process updates its
      <structname>pg_stat_activity</structname>.<structfield>status</structfield>.
      arg0 is the new status string.</entry>
    </row>
    <row>
     <entry><literal>checkpoint-start</literal></entry>
     <entry><literal>(int)</literal></entry>
     <entry>Probe that fires when a checkpoint is started.
      arg0 holds the bitwise flags used to distinguish different checkpoint
      types, such as shutdown, immediate or force.</entry>
    </row>
    <row>
     <entry><literal>checkpoint-done</literal></entry>
     <entry><literal>(int, int, int, int, int)</literal></entry>
     <entry>Probe that fires when a checkpoint is complete.
      (The probes listed next fire in sequence during checkpoint processing.)
      arg0 is the number of buffers written. arg1 is the total number of
      buffers. arg2, arg3 and arg4 contain the number of WAL files added,
      removed and recycled respectively.</entry>
    </row>
    <row>
     <entry><literal>clog-checkpoint-start</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the CLOG portion of a checkpoint is started.
      arg0 is true for normal checkpoint, false for shutdown
      checkpoint.</entry>
    </row>
    <row>
     <entry><literal>clog-checkpoint-done</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the CLOG portion of a checkpoint is
      complete. arg0 has the same meaning as for <literal>clog-checkpoint-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>subtrans-checkpoint-start</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the SUBTRANS portion of a checkpoint is
      started.
      arg0 is true for normal checkpoint, false for shutdown
      checkpoint.</entry>
    </row>
    <row>
     <entry><literal>subtrans-checkpoint-done</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the SUBTRANS portion of a checkpoint is
      complete. arg0 has the same meaning as for
      <literal>subtrans-checkpoint-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>multixact-checkpoint-start</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the MultiXact portion of a checkpoint is
      started.
      arg0 is true for normal checkpoint, false for shutdown
      checkpoint.</entry>
    </row>
    <row>
     <entry><literal>multixact-checkpoint-done</literal></entry>
     <entry><literal>(bool)</literal></entry>
     <entry>Probe that fires when the MultiXact portion of a checkpoint is
      complete. arg0 has the same meaning as for
      <literal>multixact-checkpoint-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>buffer-checkpoint-start</literal></entry>
     <entry><literal>(int)</literal></entry>
     <entry>Probe that fires when the buffer-writing portion of a checkpoint
      is started.
      arg0 holds the bitwise flags used to distinguish different checkpoint
      types, such as shutdown, immediate or force.</entry>
    </row>
    <row>
     <entry><literal>buffer-sync-start</literal></entry>
     <entry><literal>(int, int)</literal></entry>
     <entry>Probe that fires when we begin to write dirty buffers during
      checkpoint (after identifying which buffers must be written).
      arg0 is the total number of buffers.
      arg1 is the number that are currently dirty and need to be written.</entry>
    </row>
    <row>
     <entry><literal>buffer-sync-written</literal></entry>
     <entry><literal>(int)</literal></entry>
     <entry>Probe that fires after each buffer is written during checkpoint.
      arg0 is the ID number of the buffer.</entry>
    </row>
    <row>
     <entry><literal>buffer-sync-done</literal></entry>
     <entry><literal>(int, int, int)</literal></entry>
     <entry>Probe that fires when all dirty buffers have been written.
      arg0 is the total number of buffers.
      arg1 is the number of buffers actually written by the checkpoint process.
      arg2 is the number that were expected to be written (arg1 of
      <literal>buffer-sync-start</literal>); any difference reflects other processes flushing
      buffers during the checkpoint.</entry>
    </row>
    <row>
     <entry><literal>buffer-checkpoint-sync-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires after dirty buffers have been written to the
      kernel, and before starting to issue fsync requests.</entry>
    </row>
    <row>
     <entry><literal>buffer-checkpoint-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when syncing of buffers to disk is
      complete.</entry>
    </row>
    <row>
     <entry><literal>twophase-checkpoint-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the two-phase portion of a checkpoint is
      started.</entry>
    </row>
    <row>
     <entry><literal>twophase-checkpoint-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when the two-phase portion of a checkpoint is
      complete.</entry>
    </row>
    <row>
     <entry><literal>buffer-extend-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, unsigned int)</literal></entry>
     <entry>Probe that fires when a relation extension starts.
       arg0 contains the fork to be extended. arg1, arg2, and arg3 contain the
       tablespace, database, and relation OIDs identifying the relation.  arg4
       is the ID of the backend which created the temporary relation for a
       local buffer, or <symbol>INVALID_PROC_NUMBER</symbol> (-1) for a shared
       buffer. arg5 is the number of blocks the caller would like to extend
       by.</entry>
    </row>
    <row>
     <entry><literal>buffer-extend-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, unsigned int, BlockNumber)</literal></entry>
     <entry>Probe that fires when a relation extension is complete.
       arg0 contains the fork to be extended. arg1, arg2, and arg3 contain the
       tablespace, database, and relation OIDs identifying the relation.  arg4
       is the ID of the backend which created the temporary relation for a
       local buffer, or <symbol>INVALID_PROC_NUMBER</symbol> (-1) for a shared
       buffer. arg5 is the number of blocks the relation was extended by, this
       can be less than the number in the
       <literal>buffer-extend-start</literal> due to resource
       constraints. arg6 contains the BlockNumber of the first new
       block.</entry>
    </row>
    <row>
     <entry><literal>buffer-read-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</literal></entry>
     <entry>Probe that fires when a buffer read is started.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>INVALID_PROC_NUMBER</symbol> (-1) for a shared buffer.
      </entry>
    </row>
    <row>
     <entry><literal>buffer-read-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool)</literal></entry>
     <entry>Probe that fires when a buffer read is complete.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>INVALID_PROC_NUMBER</symbol> (-1) for a shared buffer.
      arg6 is true if the buffer was found in the pool, false if not.</entry>
    </row>
    <row>
     <entry><literal>buffer-flush-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
     <entry>Probe that fires before issuing any write request for a shared
      buffer.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.</entry>
    </row>
    <row>
     <entry><literal>buffer-flush-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
     <entry>Probe that fires when a write request is complete.  (Note
      that this just reflects the time to pass the data to the kernel;
      it's typically not actually been written to disk yet.)
      The arguments are the same as for <literal>buffer-flush-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>wal-buffer-write-dirty-start</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when a server process begins to write a
      dirty WAL buffer because no more WAL buffer space is available.
      (If this happens often, it implies that
      <xref linkend="guc-wal-buffers"/> is too small.)</entry>
    </row>
    <row>
     <entry><literal>wal-buffer-write-dirty-done</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when a dirty WAL buffer write is complete.</entry>
    </row>
    <row>
     <entry><literal>wal-insert</literal></entry>
     <entry><literal>(unsigned char, unsigned char)</literal></entry>
     <entry>Probe that fires when a WAL record is inserted.
      arg0 is the resource manager (rmid) for the record.
      arg1 contains the info flags.</entry>
    </row>
    <row>
     <entry><literal>wal-switch</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when a WAL segment switch is requested.</entry>
    </row>
    <row>
     <entry><literal>smgr-md-read-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</literal></entry>
     <entry>Probe that fires when beginning to read a block from a relation.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>INVALID_PROC_NUMBER</symbol> (-1) for a shared buffer.</entry>
    </row>
    <row>
     <entry><literal>smgr-md-read-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</literal></entry>
     <entry>Probe that fires when a block read is complete.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>INVALID_PROC_NUMBER</symbol> (-1) for a shared buffer.
      arg6 is the number of bytes actually read, while arg7 is the number
      requested (if these are different it indicates a short read).</entry>
    </row>
    <row>
     <entry><literal>smgr-md-write-start</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</literal></entry>
     <entry>Probe that fires when beginning to write a block to a relation.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>INVALID_PROC_NUMBER</symbol> (-1) for a shared buffer.</entry>
    </row>
    <row>
     <entry><literal>smgr-md-write-done</literal></entry>
     <entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</literal></entry>
     <entry>Probe that fires when a block write is complete.
      arg0 and arg1 contain the fork and block numbers of the page.
      arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
      identifying the relation.
      arg5 is the ID of the backend which created the temporary relation for a
      local buffer, or <symbol>INVALID_PROC_NUMBER</symbol> (-1) for a shared buffer.
      arg6 is the number of bytes actually written, while arg7 is the number
      requested (if these are different it indicates a short write).</entry>
    </row>
    <row>
     <entry><literal>sort-start</literal></entry>
     <entry><literal>(int, bool, int, int, bool, int)</literal></entry>
     <entry>Probe that fires when a sort operation is started.
      arg0 indicates heap, index or datum sort.
      arg1 is true for unique-value enforcement.
      arg2 is the number of key columns.
      arg3 is the number of kilobytes of work memory allowed.
      arg4 is true if random access to the sort result is required.
      arg5 indicates serial when <literal>0</literal>, parallel worker when
      <literal>1</literal>, or parallel leader when <literal>2</literal>.</entry>
    </row>
    <row>
     <entry><literal>sort-done</literal></entry>
     <entry><literal>(bool, long)</literal></entry>
     <entry>Probe that fires when a sort is complete.
      arg0 is true for external sort, false for internal sort.
      arg1 is the number of disk blocks used for an external sort,
      or kilobytes of memory used for an internal sort.</entry>
    </row>
    <row>
     <entry><literal>lwlock-acquire</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when an LWLock has been acquired.
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lwlock-release</literal></entry>
     <entry><literal>(char *)</literal></entry>
     <entry>Probe that fires when an LWLock has been released (but note
      that any released waiters have not yet been awakened).
      arg0 is the LWLock's tranche.</entry>
    </row>
    <row>
     <entry><literal>lwlock-wait-start</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when an LWLock was not immediately available and
      a server process has begun to wait for the lock to become available.
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lwlock-wait-done</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when a server process has been released from its
      wait for an LWLock (it does not actually have the lock yet).
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lwlock-condacquire</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when an LWLock was successfully acquired when the
      caller specified no waiting.
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lwlock-condacquire-fail</literal></entry>
     <entry><literal>(char *, LWLockMode)</literal></entry>
     <entry>Probe that fires when an LWLock was not successfully acquired when
      the caller specified no waiting.
      arg0 is the LWLock's tranche.
      arg1 is the requested lock mode, either exclusive or shared.</entry>
    </row>
    <row>
     <entry><literal>lock-wait-start</literal></entry>
     <entry><literal>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</literal></entry>
     <entry>Probe that fires when a request for a heavyweight lock (lmgr lock)
      has begun to wait because the lock is not available.
      arg0 through arg3 are the tag fields identifying the object being
      locked.  arg4 indicates the type of object being locked.
      arg5 indicates the lock type being requested.</entry>
    </row>
    <row>
     <entry><literal>lock-wait-done</literal></entry>
     <entry><literal>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</literal></entry>
     <entry>Probe that fires when a request for a heavyweight lock (lmgr lock)
      has finished waiting (i.e., has acquired the lock).
      The arguments are the same as for <literal>lock-wait-start</literal>.</entry>
    </row>
    <row>
     <entry><literal>deadlock-found</literal></entry>
     <entry><literal>()</literal></entry>
     <entry>Probe that fires when a deadlock is found by the deadlock
      detector.</entry>
    </row>

   </tbody>
   </tgroup>
  </table>

 <table id="typedefs-table">
  <title>Defined Types Used in Probe Parameters</title>
  <tgroup cols="2">
   <thead>
    <row>
     <entry>Type</entry>
     <entry>Definition</entry>
    </row>
   </thead>

   <tbody>

    <row>
     <entry><type>LocalTransactionId</type></entry>
     <entry><type>unsigned int</type></entry>
    </row>
    <row>
     <entry><type>LWLockMode</type></entry>
     <entry><type>int</type></entry>
    </row>
    <row>
     <entry><type>LOCKMODE</type></entry>
     <entry><type>int</type></entry>
    </row>
    <row>
     <entry><type>BlockNumber</type></entry>
     <entry><type>unsigned int</type></entry>
    </row>
    <row>
     <entry><type>Oid</type></entry>
     <entry><type>unsigned int</type></entry>
    </row>
    <row>
     <entry><type>ForkNumber</type></entry>
     <entry><type>int</type></entry>
    </row>
    <row>
     <entry><type>bool</type></entry>
     <entry><type>unsigned char</type></entry>
    </row>

   </tbody>
   </tgroup>
  </table>


  </sect2>

  <sect2 id="using-trace-points">
   <title>Using Probes</title>

  <para>
   The example below shows a DTrace script for analyzing transaction
   counts in the system, as an alternative to snapshotting
   <structname>pg_stat_database</structname> before and after a performance test:
<programlisting>
#!/usr/sbin/dtrace -qs

postgresql$1:::transaction-start
{
      @start["Start"] = count();
      self->ts  = timestamp;
}

postgresql$1:::transaction-abort
{
      @abort["Abort"] = count();
}

postgresql$1:::transaction-commit
/self->ts/
{
      @commit["Commit"] = count();
      @time["Total time (ns)"] = sum(timestamp - self->ts);
      self->ts=0;
}
</programlisting>
   When executed, the example D script gives output such as:
<screen>
# ./txn_count.d `pgrep -n postgres` or ./txn_count.d &lt;PID&gt;
^C

Start                                          71
Commit                                         70
Total time (ns)                        2312105013
</screen>
  </para>

  <note>
   <para>
    SystemTap uses a different notation for trace scripts than DTrace does,
    even though the underlying trace points are compatible.  One point worth
    noting is that at this writing, SystemTap scripts must reference probe
    names using double underscores in place of hyphens.  This is expected to
    be fixed in future SystemTap releases.
   </para>
  </note>

  <para>
   You should remember that DTrace scripts need to be carefully written and
   debugged, otherwise the trace information collected might
   be meaningless. In most cases where problems are found it is the
   instrumentation that is at fault, not the underlying system. When
   discussing information found using dynamic tracing, be sure to enclose
   the script used to allow that too to be checked and discussed.
  </para>
  </sect2>

  <sect2 id="defining-trace-points">
   <title>Defining New Probes</title>

  <para>
   New probes can be defined within the code wherever the developer
   desires, though this will require a recompilation. Below are the steps
   for inserting new probes:
  </para>

  <procedure>
   <step>
    <para>
     Decide on probe names and data to be made available through the probes
    </para>
   </step>

   <step>
    <para>
     Add the probe definitions to <filename>src/backend/utils/probes.d</filename>
    </para>
   </step>

   <step>
    <para>
     Include <filename>pg_trace.h</filename> if it is not already present in the
     module(s) containing the probe points, and insert
     <literal>TRACE_POSTGRESQL</literal> probe macros at the desired locations
     in the source code
    </para>
   </step>

   <step>
    <para>
     Recompile and verify that the new probes are available
    </para>
   </step>
  </procedure>

  <formalpara>
   <title>Example:</title>
   <para>
    Here is an example of how you would add a probe to trace all new
    transactions by transaction ID.
   </para>
  </formalpara>

  <procedure>
   <step>
    <para>
     Decide that the probe will be named <literal>transaction-start</literal> and
     requires a parameter of type <type>LocalTransactionId</type>
    </para>
   </step>

   <step>
    <para>
     Add the probe definition to <filename>src/backend/utils/probes.d</filename>:
<programlisting>
probe transaction__start(LocalTransactionId);
</programlisting>
     Note the use of the double underline in the probe name. In a DTrace
     script using the probe, the double underline needs to be replaced with a
     hyphen, so <literal>transaction-start</literal> is the name to document for
     users.
    </para>
   </step>

   <step>
    <para>
     At compile time, <literal>transaction__start</literal> is converted to a macro
     called <literal>TRACE_POSTGRESQL_TRANSACTION_START</literal> (notice the
     underscores are single here), which is available by including
     <filename>pg_trace.h</filename>.  Add the macro call to the appropriate location
     in the source code.  In this case, it looks like the following:

<programlisting>
TRACE_POSTGRESQL_TRANSACTION_START(vxid.localTransactionId);
</programlisting>
    </para>
   </step>

   <step>
    <para>
     After recompiling and running the new binary, check that your newly added
     probe is available by executing the following DTrace command.  You
     should see similar output:
<screen>
# dtrace -ln transaction-start
   ID    PROVIDER          MODULE           FUNCTION NAME
18705 postgresql49878     postgres     StartTransactionCommand transaction-start
18755 postgresql49877     postgres     StartTransactionCommand transaction-start
18805 postgresql49876     postgres     StartTransactionCommand transaction-start
18855 postgresql49875     postgres     StartTransactionCommand transaction-start
18986 postgresql49873     postgres     StartTransactionCommand transaction-start
</screen>
    </para>
   </step>
  </procedure>

  <para>
   There are a few things to be careful about when adding trace macros
   to the C code:

   <itemizedlist>
    <listitem>
     <para>
      You should take care that the data types specified for a probe's
      parameters match the data types of the variables used in the macro.
      Otherwise, you will get compilation errors.
     </para>
    </listitem>


    <listitem>
     <para>
      On most platforms, if <productname>PostgreSQL</productname> is
      built with <option>--enable-dtrace</option>, the arguments to a trace
      macro will be evaluated whenever control passes through the
      macro, <emphasis>even if no tracing is being done</emphasis>.  This is
      usually not worth worrying about if you are just reporting the
      values of a few local variables.  But beware of putting expensive
      function calls into the arguments.  If you need to do that,
      consider protecting the macro with a check to see if the trace
      is actually enabled:

<programlisting>
if (TRACE_POSTGRESQL_TRANSACTION_START_ENABLED())
    TRACE_POSTGRESQL_TRANSACTION_START(some_function(...));
</programlisting>

      Each trace macro has a corresponding <literal>ENABLED</literal> macro.
     </para>
    </listitem>
   </itemizedlist>

  </para>

  </sect2>

 </sect1>

 <sect1 id="diskusage">
  <title>Monitoring Disk Usage</title>

  <para>
   This section discusses how to monitor the disk usage of a
   <productname>PostgreSQL</productname> database system.
  </para>

  <sect2 id="disk-usage">
   <title>Determining Disk Usage</title>

   <indexterm zone="disk-usage">
    <primary>disk usage</primary>
   </indexterm>

   <para>
    Each table has a primary heap disk file where most of the data is
    stored. If the table has any columns with potentially-wide values,
    there also might be a <acronym>TOAST</acronym> file associated with the table,
    which is used to store values too wide to fit comfortably in the main
    table (see <xref linkend="storage-toast"/>).  There will be one valid index
    on the <acronym>TOAST</acronym> table, if present. There also might be indexes
    associated with the base table.  Each table and index is stored in a
    separate disk file &mdash; possibly more than one file, if the file would
    exceed one gigabyte.  Naming conventions for these files are described
    in <xref linkend="storage-file-layout"/>.
   </para>

   <para>
    You can monitor disk space in three ways:
    using the SQL functions listed in <xref linkend="functions-admin-dbsize"/>,
    using the <xref linkend="oid2name"/> module, or
    using manual inspection of the system catalogs.
    The SQL functions are the easiest to use and are generally recommended.
    The remainder of this section shows how to do it by inspection of the
    system catalogs.
   </para>

   <para>
    Using <application>psql</application> on a recently vacuumed or analyzed
    database, you can issue queries to see the disk usage of any table:
<programlisting>
SELECT pg_relation_filepath(oid), relpages FROM pg_class WHERE relname = 'customer';

 pg_relation_filepath | relpages
----------------------+----------
 base/16384/16806     |       60
(1 row)
</programlisting>
    Each page is typically 8 kilobytes. (Remember, <structfield>relpages</structfield>
    is only updated by <command>VACUUM</command>, <command>ANALYZE</command>, and
    a few DDL commands such as <command>CREATE INDEX</command>.)  The file path name
    is of interest if you want to examine the table's disk file directly.
   </para>

   <para>
    To show the space used by <acronym>TOAST</acronym> tables, use a query
    like the following:
<programlisting>
SELECT relname, relpages
FROM pg_class,
     (SELECT reltoastrelid
      FROM pg_class
      WHERE relname = 'customer') AS ss
WHERE oid = ss.reltoastrelid OR
      oid = (SELECT indexrelid
             FROM pg_index
             WHERE indrelid = ss.reltoastrelid)
ORDER BY relname;

       relname        | relpages
----------------------+----------
 pg_toast_16806       |        0
 pg_toast_16806_index |        1
</programlisting>
   </para>

   <para>
    You can easily display index sizes, too:
<programlisting>
SELECT c2.relname, c2.relpages
FROM pg_class c, pg_class c2, pg_index i
WHERE c.relname = 'customer' AND
      c.oid = i.indrelid AND
      c2.oid = i.indexrelid
ORDER BY c2.relname;

      relname      | relpages
-------------------+----------
 customer_id_index |       26
</programlisting>
   </para>

   <para>
    It is easy to find your largest tables and indexes using this
    information:
<programlisting>
SELECT relname, relpages
FROM pg_class
ORDER BY relpages DESC;

       relname        | relpages
----------------------+----------
 bigtable             |     3290
 customer             |     3144
</programlisting>
   </para>
  </sect2>

  <sect2 id="disk-full">
   <title>Disk Full Failure</title>

   <para>
    The most important disk monitoring task of a database administrator
    is to make sure the disk doesn't become full.  A filled data disk will
    not result in data corruption, but it might prevent useful activity
    from occurring. If the disk holding the WAL files grows full, database
    server panic and consequent shutdown might occur.
   </para>

   <para>
    If you cannot free up additional space on the disk by deleting
    other things, you can move some of the database files to other file
    systems by making use of tablespaces. See <xref
    linkend="manage-ag-tablespaces"/> for more information about that.
   </para>

   <tip>
    <para>
     Some file systems perform badly when they are almost full, so do
     not wait until the disk is completely full to take action.
    </para>
   </tip>

   <para>
    If your system supports per-user disk quotas, then the database
    will naturally be subject to whatever quota is placed on the user
    the server runs as.  Exceeding the quota will have the same bad
    effects as running out of disk space entirely.
   </para>
  </sect2>
 </sect1>

</chapter>