postgresql/doc/src/sgml/tsm-system-rows.sgml

<!-- doc/src/sgml/tsm-system-rows.sgml -->

<sect1 id="tsm-system-rows" xreflabel="tsm_system_rows">
 <title>tsm_system_rows &mdash;
   the <literal>SYSTEM_ROWS</literal> sampling method for <literal>TABLESAMPLE</literal></title>

 <indexterm zone="tsm-system-rows">
  <primary>tsm_system_rows</primary>
 </indexterm>

 <para>
  The <filename>tsm_system_rows</filename> module provides the table sampling method
  <literal>SYSTEM_ROWS</literal>, which can be used in
  the <literal>TABLESAMPLE</literal> clause of a <link linkend="sql-select"><command>SELECT</command></link>
  command.
 </para>

 <para>
  This table sampling method accepts a single integer argument that is the
  maximum number of rows to read.  The resulting sample will always contain
  exactly that many rows, unless the table does not contain enough rows, in
  which case the whole table is selected.
 </para>

 <para>
  Like the built-in <literal>SYSTEM</literal> sampling
  method, <literal>SYSTEM_ROWS</literal> performs block-level sampling, so
  that the sample is not completely random but may be subject to clustering
  effects, especially if only a small number of rows are requested.
 </para>

 <para>
  <literal>SYSTEM_ROWS</literal> does not support
  the <literal>REPEATABLE</literal> clause.
 </para>

 <para>
  This module is considered <quote>trusted</quote>, that is, it can be
  installed by non-superusers who have <literal>CREATE</literal> privilege
  on the current database.
 </para>

 <sect2 id="tsm-system-rows-examples">
  <title>Examples</title>

  <para>
   Here is an example of selecting a sample of a table with
   <literal>SYSTEM_ROWS</literal>.  First install the extension:
  </para>

<programlisting>
CREATE EXTENSION tsm_system_rows;
</programlisting>

  <para>
   Then you can use it in a <command>SELECT</command> command, for instance:

<programlisting>
SELECT * FROM my_table TABLESAMPLE SYSTEM_ROWS(100);
</programlisting>
  </para>

  <para>
   This command will return a sample of 100 rows from the
   table <structname>my_table</structname> (unless the table does not have 100
   visible rows, in which case all its rows are returned).
  </para>
 </sect2>

</sect1>