doc/src/sgml/ref/vacuum.sgml

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   2 doc/src/sgml/ref/vacuum.sgml
   3 PostgreSQL documentation
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   5
   6 <refentry id="sql-vacuum">
   7  <indexterm zone="sql-vacuum">
   8   <primary>VACUUM</primary>
   9  </indexterm>
  10
  11  <refmeta>
  12   <refentrytitle>VACUUM</refentrytitle>
  13   <manvolnum>7</manvolnum>
  14   <refmiscinfo>SQL - Language Statements</refmiscinfo>
  15  </refmeta>
  16
  17  <refnamediv>
  18   <refname>VACUUM</refname>
  19   <refpurpose>garbage-collect and optionally analyze a database</refpurpose>
  20  </refnamediv>
  21
  22  <refsynopsisdiv>
  23 <synopsis>
  24 VACUUM [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] [ <replaceable class="parameter">table_and_columns</replaceable> [, ...] ]
  25
  26 <phrase>where <replaceable class="parameter">option</replaceable> can be one of:</phrase>
  27
  28     FULL [ <replaceable class="parameter">boolean</replaceable> ]
  29     FREEZE [ <replaceable class="parameter">boolean</replaceable> ]
  30     VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
  31     ANALYZE [ <replaceable class="parameter">boolean</replaceable> ]
  32     DISABLE_PAGE_SKIPPING [ <replaceable class="parameter">boolean</replaceable> ]
  33     SKIP_LOCKED [ <replaceable class="parameter">boolean</replaceable> ]
  34     INDEX_CLEANUP { AUTO | ON | OFF }
  35     PROCESS_MAIN [ <replaceable class="parameter">boolean</replaceable> ]
  36     PROCESS_TOAST [ <replaceable class="parameter">boolean</replaceable> ]
  37     TRUNCATE [ <replaceable class="parameter">boolean</replaceable> ]
  38     PARALLEL <replaceable class="parameter">integer</replaceable>
  39     SKIP_DATABASE_STATS [ <replaceable class="parameter">boolean</replaceable> ]
  40     ONLY_DATABASE_STATS [ <replaceable class="parameter">boolean</replaceable> ]
  41     BUFFER_USAGE_LIMIT <replaceable class="parameter">size</replaceable>
  42
  43 <phrase>and <replaceable class="parameter">table_and_columns</replaceable> is:</phrase>
  44
  45     [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
  46 </synopsis>
  47  </refsynopsisdiv>
  48
  49  <refsect1>
  50   <title>Description</title>
  51
  52   <para>
  53    <command>VACUUM</command> reclaims storage occupied by dead tuples.
  54    In normal <productname>PostgreSQL</productname> operation, tuples that
  55    are deleted or obsoleted by an update are not physically removed from
  56    their table; they remain present until a <command>VACUUM</command> is
  57    done.  Therefore it's necessary to do <command>VACUUM</command>
  58    periodically, especially on frequently-updated tables.
  59   </para>
  60
  61   <para>
  62    Without a <replaceable class="parameter">table_and_columns</replaceable>
  63    list, <command>VACUUM</command> processes every table and materialized view
  64    in the current database that the current user has permission to vacuum.
  65    With a list, <command>VACUUM</command> processes only those table(s).
  66   </para>
  67
  68   <para>
  69    <command>VACUUM ANALYZE</command> performs a <command>VACUUM</command>
  70    and then an <command>ANALYZE</command> for each selected table.  This
  71    is a handy combination form for routine maintenance scripts.  See
  72    <xref linkend="sql-analyze"/>
  73    for more details about its processing.
  74   </para>
  75
  76   <para>
  77    Plain <command>VACUUM</command> (without <literal>FULL</literal>) simply reclaims
  78    space and makes it
  79    available for re-use.  This form of the command can operate in parallel
  80    with normal reading and writing of the table, as an exclusive lock
  81    is not obtained.  However, extra space is not returned to the operating
  82    system (in most cases); it's just kept available for re-use within the
  83    same table.  It also allows us to leverage multiple CPUs in order to process
  84    indexes.  This feature is known as <firstterm>parallel vacuum</firstterm>.
  85    To disable this feature, one can use <literal>PARALLEL</literal> option and
  86    specify parallel workers as zero.  <command>VACUUM FULL</command> rewrites
  87    the entire contents of the table into a new disk file with no extra space,
  88    allowing unused space to be returned to the operating system.  This form is
  89    much slower and requires an <literal>ACCESS EXCLUSIVE</literal> lock on
  90    each table while it is being processed.
  91   </para>
  92  </refsect1>
  93
  94  <refsect1>
  95   <title>Parameters</title>
  96
  97   <variablelist>
  98    <varlistentry>
  99     <term><literal>FULL</literal></term>
 100     <listitem>
 101      <para>
 102       Selects <quote>full</quote> vacuum, which can reclaim more
 103       space, but takes much longer and exclusively locks the table.
 104       This method also requires extra disk space, since it writes a
 105       new copy of the table and doesn't release the old copy until
 106       the operation is complete.  Usually this should only be used when a
 107       significant amount of space needs to be reclaimed from within the table.
 108      </para>
 109     </listitem>
 110    </varlistentry>
 111
 112    <varlistentry>
 113     <term><literal>FREEZE</literal></term>
 114     <listitem>
 115      <para>
 116       Selects aggressive <quote>freezing</quote> of tuples.
 117       Specifying <literal>FREEZE</literal> is equivalent to performing
 118       <command>VACUUM</command> with the
 119       <xref linkend="guc-vacuum-freeze-min-age"/> and
 120       <xref linkend="guc-vacuum-freeze-table-age"/> parameters
 121       set to zero.  Aggressive freezing is always performed when the
 122       table is rewritten, so this option is redundant when <literal>FULL</literal>
 123       is specified.
 124      </para>
 125     </listitem>
 126    </varlistentry>
 127
 128    <varlistentry>
 129     <term><literal>VERBOSE</literal></term>
 130     <listitem>
 131      <para>
 132       Prints a detailed vacuum activity report for each table
 133       at <literal>INFO</literal> level.
 134      </para>
 135     </listitem>
 136    </varlistentry>
 137
 138    <varlistentry>
 139     <term><literal>ANALYZE</literal></term>
 140     <listitem>
 141      <para>
 142       Updates statistics used by the planner to determine the most
 143       efficient way to execute a query.
 144      </para>
 145     </listitem>
 146    </varlistentry>
 147
 148    <varlistentry>
 149     <term><literal>DISABLE_PAGE_SKIPPING</literal></term>
 150     <listitem>
 151      <para>
 152       Normally, <command>VACUUM</command> will skip pages based on the <link
 153       linkend="vacuum-for-visibility-map">visibility map</link>.  Pages where
 154       all tuples are known to be frozen can always be skipped, and those
 155       where all tuples are known to be visible to all transactions may be
 156       skipped except when performing an aggressive vacuum.  Furthermore,
 157       except when performing an aggressive vacuum, some pages may be skipped
 158       in order to avoid waiting for other sessions to finish using them.
 159       This option disables all page-skipping behavior, and is intended to
 160       be used only when the contents of the visibility map are
 161       suspect, which should happen only if there is a hardware or software
 162       issue causing database corruption.
 163      </para>
 164     </listitem>
 165    </varlistentry>
 166
 167    <varlistentry>
 168     <term><literal>SKIP_LOCKED</literal></term>
 169     <listitem>
 170      <para>
 171       Specifies that <command>VACUUM</command> should not wait for any
 172       conflicting locks to be released when beginning work on a relation:
 173       if a relation cannot be locked immediately without waiting, the relation
 174       is skipped.  Note that even with this option,
 175       <command>VACUUM</command> may still block when opening the relation's
 176       indexes.  Additionally, <command>VACUUM ANALYZE</command> may still
 177       block when acquiring sample rows from partitions, table inheritance
 178       children, and some types of foreign tables.  Also, while
 179       <command>VACUUM</command> ordinarily processes all partitions of
 180       specified partitioned tables, this option will cause
 181       <command>VACUUM</command> to skip all partitions if there is a
 182       conflicting lock on the partitioned table.
 183      </para>
 184     </listitem>
 185    </varlistentry>
 186
 187    <varlistentry>
 188     <term><literal>INDEX_CLEANUP</literal></term>
 189     <listitem>
 190      <para>
 191       Normally, <command>VACUUM</command> will skip index vacuuming
 192       when there are very few dead tuples in the table.  The cost of
 193       processing all of the table's indexes is expected to greatly
 194       exceed the benefit of removing dead index tuples when this
 195       happens.  This option can be used to force
 196       <command>VACUUM</command> to process indexes when there are more
 197       than zero dead tuples.  The default is <literal>AUTO</literal>,
 198       which allows <command>VACUUM</command> to skip index vacuuming
 199       when appropriate.  If <literal>INDEX_CLEANUP</literal> is set to
 200       <literal>ON</literal>, <command>VACUUM</command> will
 201       conservatively remove all dead tuples from indexes.  This may be
 202       useful for backwards compatibility with earlier releases of
 203       <productname>PostgreSQL</productname> where this was the
 204       standard behavior.
 205      </para>
 206      <para>
 207       <literal>INDEX_CLEANUP</literal> can also be set to
 208       <literal>OFF</literal> to force <command>VACUUM</command> to
 209       <emphasis>always</emphasis> skip index vacuuming, even when
 210       there are many dead tuples in the table.  This may be useful
 211       when it is necessary to make <command>VACUUM</command> run as
 212       quickly as possible to avoid imminent transaction ID wraparound
 213       (see <xref linkend="vacuum-for-wraparound"/>).  However, the
 214       wraparound failsafe mechanism controlled by <xref
 215        linkend="guc-vacuum-failsafe-age"/>  will generally trigger
 216       automatically to avoid transaction ID wraparound failure, and
 217       should be preferred.  If index cleanup is not performed
 218       regularly, performance may suffer, because as the table is
 219       modified indexes will accumulate dead tuples and the table
 220       itself will accumulate dead line pointers that cannot be removed
 221       until index cleanup is completed.
 222      </para>
 223      <para>
 224       This option has no effect for tables that have no index and is
 225       ignored if the <literal>FULL</literal> option is used.  It also
 226       has no effect on the transaction ID wraparound failsafe
 227       mechanism.  When triggered it will skip index vacuuming, even
 228       when <literal>INDEX_CLEANUP</literal> is set to
 229       <literal>ON</literal>.
 230      </para>
 231     </listitem>
 232    </varlistentry>
 233
 234    <varlistentry>
 235     <term><literal>PROCESS_MAIN</literal></term>
 236     <listitem>
 237      <para>
 238       Specifies that <command>VACUUM</command> should attempt to process the
 239       main relation. This is usually the desired behavior and is the default.
 240       Setting this option to false may be useful when it is only necessary to
 241       vacuum a relation's corresponding <literal>TOAST</literal> table.
 242      </para>
 243     </listitem>
 244    </varlistentry>
 245
 246    <varlistentry>
 247     <term><literal>PROCESS_TOAST</literal></term>
 248     <listitem>
 249      <para>
 250       Specifies that <command>VACUUM</command> should attempt to process the
 251       corresponding <literal>TOAST</literal> table for each relation, if one
 252       exists. This is usually the desired behavior and is the default.
 253       Setting this option to false may be useful when it is only necessary to
 254       vacuum the main relation. This option is required when the
 255       <literal>FULL</literal> option is used.
 256      </para>
 257     </listitem>
 258    </varlistentry>
 259
 260    <varlistentry>
 261     <term><literal>TRUNCATE</literal></term>
 262     <listitem>
 263      <para>
 264       Specifies that <command>VACUUM</command> should attempt to
 265       truncate off any empty pages at the end of the table and allow
 266       the disk space for the truncated pages to be returned to
 267       the operating system. This is normally the desired behavior
 268       and is the default unless the <literal>vacuum_truncate</literal>
 269       option has been set to false for the table to be vacuumed.
 270       Setting this option to false may be useful to avoid
 271       <literal>ACCESS EXCLUSIVE</literal> lock on the table that
 272       the truncation requires. This option is ignored if the
 273       <literal>FULL</literal> option is used.
 274      </para>
 275     </listitem>
 276    </varlistentry>
 277
 278    <varlistentry>
 279     <term><literal>PARALLEL</literal></term>
 280     <listitem>
 281      <para>
 282       Perform index vacuum and index cleanup phases of <command>VACUUM</command>
 283       in parallel using <replaceable class="parameter">integer</replaceable>
 284       background workers (for the details of each vacuum phase, please
 285       refer to <xref linkend="vacuum-phases"/>).  The number of workers used
 286       to perform the operation is equal to the number of indexes on the
 287       relation that support parallel vacuum which is limited by the number of
 288       workers specified with <literal>PARALLEL</literal> option if any which is
 289       further limited by <xref linkend="guc-max-parallel-maintenance-workers"/>.
 290       An index can participate in parallel vacuum if and only if the size of the
 291       index is more than <xref linkend="guc-min-parallel-index-scan-size"/>.
 292       Please note that it is not guaranteed that the number of parallel workers
 293       specified in <replaceable class="parameter">integer</replaceable> will be
 294       used during execution.  It is possible for a vacuum to run with fewer
 295       workers than specified, or even with no workers at all.  Only one worker
 296       can be used per index.  So parallel workers are launched only when there
 297       are at least <literal>2</literal> indexes in the table.  Workers for
 298       vacuum are launched before the start of each phase and exit at the end of
 299       the phase.  These behaviors might change in a future release.  This
 300       option can't be used with the <literal>FULL</literal> option.
 301      </para>
 302     </listitem>
 303    </varlistentry>
 304
 305    <varlistentry>
 306     <term><literal>SKIP_DATABASE_STATS</literal></term>
 307     <listitem>
 308      <para>
 309       Specifies that <command>VACUUM</command> should skip updating the
 310       database-wide statistics about oldest unfrozen XIDs.  Normally
 311       <command>VACUUM</command> will update these statistics once at the
 312       end of the command.  However, this can take awhile in a database
 313       with a very large number of tables, and it will accomplish nothing
 314       unless the table that had contained the oldest unfrozen XID was
 315       among those vacuumed.  Moreover, if multiple <command>VACUUM</command>
 316       commands are issued in parallel, only one of them can update the
 317       database-wide statistics at a time.  Therefore, if an application
 318       intends to issue a series of many <command>VACUUM</command>
 319       commands, it can be helpful to set this option in all but the last
 320       such command; or set it in all the commands and separately
 321       issue <literal>VACUUM (ONLY_DATABASE_STATS)</literal> afterwards.
 322      </para>
 323     </listitem>
 324    </varlistentry>
 325
 326    <varlistentry>
 327     <term><literal>ONLY_DATABASE_STATS</literal></term>
 328     <listitem>
 329      <para>
 330       Specifies that <command>VACUUM</command> should do nothing except
 331       update the database-wide statistics about oldest unfrozen XIDs.
 332       When this option is specified,
 333       the <replaceable class="parameter">table_and_columns</replaceable>
 334       list must be empty, and no other option may be enabled
 335       except <literal>VERBOSE</literal>.
 336      </para>
 337     </listitem>
 338    </varlistentry>
 339
 340    <varlistentry>
 341     <term><literal>BUFFER_USAGE_LIMIT</literal></term>
 342     <listitem>
 343      <para>
 344       Specifies the
 345       <glossterm linkend="glossary-buffer-access-strategy">Buffer Access Strategy</glossterm>
 346       ring buffer size for <command>VACUUM</command>.  This size is used to
 347       calculate the number of shared buffers which will be reused as part of
 348       this strategy.  <literal>0</literal> disables use of a
 349       <literal>Buffer Access Strategy</literal>.  If <option>ANALYZE</option>
 350       is also specified, the <option>BUFFER_USAGE_LIMIT</option> value is used
 351       for both the vacuum and analyze stages.  This option can't be used with
 352       the <option>FULL</option> option except if <option>ANALYZE</option> is
 353       also specified.  When this option is not specified,
 354       <command>VACUUM</command> uses the value from
 355       <xref linkend="guc-vacuum-buffer-usage-limit"/>.  Higher settings can
 356       allow <command>VACUUM</command> to run more quickly, but having too
 357       large a setting may cause too many other useful pages to be evicted from
 358       shared buffers.  The minimum value is <literal>128 kB</literal> and the
 359       maximum value is <literal>16 GB</literal>.
 360      </para>
 361     </listitem>
 362    </varlistentry>
 363
 364    <varlistentry>
 365     <term><replaceable class="parameter">boolean</replaceable></term>
 366     <listitem>
 367      <para>
 368       Specifies whether the selected option should be turned on or off.
 369       You can write <literal>TRUE</literal>, <literal>ON</literal>, or
 370       <literal>1</literal> to enable the option, and <literal>FALSE</literal>,
 371       <literal>OFF</literal>, or <literal>0</literal> to disable it.  The
 372       <replaceable class="parameter">boolean</replaceable> value can also
 373       be omitted, in which case <literal>TRUE</literal> is assumed.
 374      </para>
 375     </listitem>
 376    </varlistentry>
 377
 378    <varlistentry>
 379     <term><replaceable class="parameter">integer</replaceable></term>
 380     <listitem>
 381      <para>
 382       Specifies a non-negative integer value passed to the selected option.
 383      </para>
 384     </listitem>
 385    </varlistentry>
 386
 387    <varlistentry>
 388     <term><replaceable class="parameter">size</replaceable></term>
 389     <listitem>
 390      <para>
 391       Specifies an amount of memory in kilobytes.  Sizes may also be specified
 392       as a string containing the numerical size followed by any one of the
 393       following memory units: <literal>B</literal> (bytes),
 394       <literal>kB</literal> (kilobytes), <literal>MB</literal> (megabytes),
 395       <literal>GB</literal> (gigabytes), or <literal>TB</literal> (terabytes).
 396      </para>
 397     </listitem>
 398    </varlistentry>
 399
 400    <varlistentry>
 401     <term><replaceable class="parameter">table_name</replaceable></term>
 402     <listitem>
 403      <para>
 404       The name (optionally schema-qualified) of a specific table or
 405       materialized view to vacuum.  If <literal>ONLY</literal> is specified
 406       before the table name, only that table is vacuumed.  If
 407       <literal>ONLY</literal> is not specified, the table and all its
 408       inheritance child tables or partitions (if any) are also vacuumed.
 409       Optionally, <literal>*</literal> can be specified after the table name
 410       to explicitly indicate that inheritance child tables (or partitions) are
 411       to be vacuumed.
 412      </para>
 413     </listitem>
 414    </varlistentry>
 415
 416    <varlistentry>
 417     <term><replaceable class="parameter">column_name</replaceable></term>
 418     <listitem>
 419      <para>
 420       The name of a specific column to analyze. Defaults to all columns.
 421       If a column list is specified, <literal>ANALYZE</literal> must also be
 422       specified.
 423      </para>
 424     </listitem>
 425    </varlistentry>
 426   </variablelist>
 427  </refsect1>
 428
 429  <refsect1>
 430   <title>Outputs</title>
 431
 432    <para>
 433     When <literal>VERBOSE</literal> is specified, <command>VACUUM</command> emits
 434     progress messages to indicate which table is currently being
 435     processed.  Various statistics about the tables are printed as well.
 436    </para>
 437  </refsect1>
 438
 439  <refsect1>
 440   <title>Notes</title>
 441
 442    <para>
 443     To vacuum a table, one must ordinarily have the <literal>MAINTAIN</literal>
 444     privilege on the table.  However, database owners are allowed to
 445     vacuum all tables in their databases, except shared catalogs.
 446     <command>VACUUM</command> will skip over any tables that the calling user
 447     does not have permission to vacuum.
 448    </para>
 449
 450    <para>
 451     While <command>VACUUM</command> is running, the <xref
 452     linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
 453     pg_temp</literal>.
 454    </para>
 455
 456    <para>
 457     <command>VACUUM</command> cannot be executed inside a transaction block.
 458    </para>
 459
 460    <para>
 461     For tables with <acronym>GIN</acronym> indexes, <command>VACUUM</command> (in
 462     any form) also completes any pending index insertions, by moving pending
 463     index entries to the appropriate places in the main <acronym>GIN</acronym> index
 464     structure.  See <xref linkend="gin-fast-update"/> for details.
 465    </para>
 466
 467    <para>
 468     We recommend that all databases be vacuumed regularly in
 469     order to remove dead rows.  <productname>PostgreSQL</productname> includes
 470     an <quote>autovacuum</quote> facility which can automate routine vacuum
 471     maintenance.  For more information about automatic and manual vacuuming,
 472     see <xref linkend="routine-vacuuming"/>.
 473    </para>
 474
 475    <para>
 476     The <option>FULL</option> option is not recommended for routine use,
 477     but might be useful in special cases.  An example is when you have deleted
 478     or updated most of the rows in a table and would like the table to
 479     physically shrink to occupy less disk space and allow faster table
 480     scans. <command>VACUUM FULL</command> will usually shrink the table
 481     more than a plain <command>VACUUM</command> would.
 482    </para>
 483
 484    <para>
 485      The <option>PARALLEL</option> option is used only for vacuum purposes.
 486      If this option is specified with the <option>ANALYZE</option> option,
 487      it does not affect <option>ANALYZE</option>.
 488    </para>
 489
 490    <para>
 491     <command>VACUUM</command> causes a substantial increase in I/O traffic,
 492     which might cause poor performance for other active sessions.  Therefore,
 493     it is sometimes advisable to use the cost-based vacuum delay feature.  For
 494     parallel vacuum, each worker sleeps in proportion to the work done by that
 495     worker.  See <xref linkend="runtime-config-resource-vacuum-cost"/> for
 496     details.
 497    </para>
 498
 499    <para>
 500     Each backend running <command>VACUUM</command> without the
 501     <literal>FULL</literal> option will report its progress in the
 502     <structname>pg_stat_progress_vacuum</structname> view. Backends running
 503     <command>VACUUM FULL</command> will instead report their progress in the
 504     <structname>pg_stat_progress_cluster</structname> view. See
 505     <xref linkend="vacuum-progress-reporting"/> and
 506     <xref linkend="cluster-progress-reporting"/> for details.
 507    </para>
 508  </refsect1>
 509
 510  <refsect1>
 511   <title>Examples</title>
 512
 513   <para>
 514    To clean a single table <literal>onek</literal>, analyze it for
 515    the optimizer and print a detailed vacuum activity report:
 516
 517 <programlisting>
 518 VACUUM (VERBOSE, ANALYZE) onek;
 519 </programlisting></para>
 520  </refsect1>
 521
 522  <refsect1>
 523   <title>Compatibility</title>
 524
 525   <para>
 526    There is no <command>VACUUM</command> statement in the SQL standard.
 527   </para>
 528
 529   <para>
 530    The following syntax was used before <productname>PostgreSQL</productname>
 531    version 9.0 and is still supported:
 532 <synopsis>
 533 VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ <replaceable class="parameter">table_and_columns</replaceable> [, ...] ]
 534 </synopsis>
 535    Note that in this syntax, the options must be specified in exactly the order
 536    shown.
 537   </para>
 538  </refsect1>
 539
 540  <refsect1>
 541   <title>See Also</title>
 542
 543   <simplelist type="inline">
 544    <member><xref linkend="app-vacuumdb"/></member>
 545    <member><xref linkend="runtime-config-resource-vacuum-cost"/></member>
 546    <member><xref linkend="autovacuum"/></member>
 547    <member><xref linkend="vacuum-progress-reporting"/></member>
 548    <member><xref linkend="cluster-progress-reporting"/></member>
 549   </simplelist>
 550  </refsect1>
 551 </refentry>