doc/src/sgml/tsm-system-time.sgml

   1 <!-- doc/src/sgml/tsm-system-time.sgml -->
   2
   3 <sect1 id="tsm-system-time" xreflabel="tsm_system_time">
   4  <title>tsm_system_time &mdash;
   5    the <literal>SYSTEM_TIME</literal> sampling method for <literal>TABLESAMPLE</literal></title>
   6
   7  <indexterm zone="tsm-system-time">
   8   <primary>tsm_system_time</primary>
   9  </indexterm>
  10
  11  <para>
  12   The <filename>tsm_system_time</filename> module provides the table sampling method
  13   <literal>SYSTEM_TIME</literal>, which can be used in
  14   the <literal>TABLESAMPLE</literal> clause of a <link linkend="sql-select"><command>SELECT</command></link>
  15   command.
  16  </para>
  17
  18  <para>
  19   This table sampling method accepts a single floating-point argument that
  20   is the maximum number of milliseconds to spend reading the table.  This
  21   gives you direct control over how long the query takes, at the price that
  22   the size of the sample becomes hard to predict.  The resulting sample will
  23   contain as many rows as could be read in the specified time, unless the
  24   whole table has been read first.
  25  </para>
  26
  27  <para>
  28   Like the built-in <literal>SYSTEM</literal> sampling
  29   method, <literal>SYSTEM_TIME</literal> performs block-level sampling, so
  30   that the sample is not completely random but may be subject to clustering
  31   effects, especially if only a small number of rows are selected.
  32  </para>
  33
  34  <para>
  35   <literal>SYSTEM_TIME</literal> does not support
  36   the <literal>REPEATABLE</literal> clause.
  37  </para>
  38
  39  <para>
  40   This module is considered <quote>trusted</quote>, that is, it can be
  41   installed by non-superusers who have <literal>CREATE</literal> privilege
  42   on the current database.
  43  </para>
  44
  45  <sect2 id="tsm-system-time-examples">
  46   <title>Examples</title>
  47
  48   <para>
  49    Here is an example of selecting a sample of a table with
  50    <literal>SYSTEM_TIME</literal>.  First install the extension:
  51   </para>
  52
  53 <programlisting>
  54 CREATE EXTENSION tsm_system_time;
  55 </programlisting>
  56
  57   <para>
  58    Then you can use it in a <command>SELECT</command> command, for instance:
  59
  60 <programlisting>
  61 SELECT * FROM my_table TABLESAMPLE SYSTEM_TIME(1000);
  62 </programlisting>
  63   </para>
  64
  65   <para>
  66    This command will return as large a sample of <structname>my_table</structname> as
  67    it can read in 1 second (1000 milliseconds).  Of course, if the whole
  68    table can be read in under 1 second, all its rows will be returned.
  69   </para>
  70  </sect2>
  71
  72 </sect1>