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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>F.46. tsm_system_rows</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="test-decoding.html" title="F.45. test_decoding" /><link rel="next" href="tsm-system-time.html" title="F.47. tsm_system_time" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.46. tsm_system_rows</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="test-decoding.html" title="F.45. test_decoding">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="tsm-system-time.html" title="F.47. tsm_system_time">Next</a></td></tr></table><hr /></div><div class="sect1" id="TSM-SYSTEM-ROWS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.46. tsm_system_rows</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="tsm-system-rows.html#id-1.11.7.55.8">F.46.1. Examples</a></span></dt></dl></div><a id="id-1.11.7.55.2" class="indexterm"></a><p>
The <code class="filename">tsm_system_rows</code> module provides the table sampling method
<code class="literal">SYSTEM_ROWS</code>, which can be used in
the <code class="literal">TABLESAMPLE</code> clause of a <a class="link" href="sql-select.html" title="SELECT"><code class="command">SELECT</code></a>
command.
</p><p>
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.
</p><p>
Like the built-in <code class="literal">SYSTEM</code> sampling
method, <code class="literal">SYSTEM_ROWS</code> 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.
</p><p>
<code class="literal">SYSTEM_ROWS</code> does not support
the <code class="literal">REPEATABLE</code> clause.
</p><p>
This module is considered <span class="quote">“<span class="quote">trusted</span>”</span>, that is, it can be
installed by non-superusers who have <code class="literal">CREATE</code> privilege
on the current database.
</p><div class="sect2" id="id-1.11.7.55.8"><div class="titlepage"><div><div><h3 class="title">F.46.1. Examples</h3></div></div></div><p>
Here is an example of selecting a sample of a table with
<code class="literal">SYSTEM_ROWS</code>. First install the extension:
</p><pre class="programlisting">
CREATE EXTENSION tsm_system_rows;
</pre><p>
Then you can use it in a <code class="command">SELECT</code> command, for instance:
</p><pre class="programlisting">
SELECT * FROM my_table TABLESAMPLE SYSTEM_ROWS(100);
</pre><p>
</p><p>
This command will return a sample of 100 rows from the
table <code class="structname">my_table</code> (unless the table does not have 100
visible rows, in which case all its rows are returned).
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="test-decoding.html" title="F.45. test_decoding">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tsm-system-time.html" title="F.47. tsm_system_time">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.45. test_decoding </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="40%" align="right" valign="top"> F.47. tsm_system_time</td></tr></table></div></body></html>
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