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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.41. 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 V1.79.1" /><link rel="prev" href="test-decoding.html" title="F.40. test_decoding" /><link rel="next" href="tsm-system-time.html" title="F.42. tsm_system_time" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.41. tsm_system_rows</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="test-decoding.html" title="F.40. 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 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="tsm-system-time.html" title="F.42. tsm_system_time">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="TSM-SYSTEM-ROWS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.41. tsm_system_rows</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="tsm-system-rows.html#id-1.11.7.50.7">F.41.1. Examples</a></span></dt></dl></div><a id="id-1.11.7.50.2" 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="xref" href="sql-select.html" title="SELECT"><span class="refentrytitle">SELECT</span></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><div class="sect2" id="id-1.11.7.50.7"><div class="titlepage"><div><div><h3 class="title">F.41.1. 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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tsm-system-time.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.40. test_decoding </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> F.42. tsm_system_time</td></tr></table></div></body></html>
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