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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.42. tsm_system_time</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="tsm-system-rows.html" title="F.41. tsm_system_rows" /><link rel="next" href="unaccent.html" title="F.43. unaccent" /></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.42. tsm_system_time</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tsm-system-rows.html" title="F.41. tsm_system_rows">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="unaccent.html" title="F.43. unaccent">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="TSM-SYSTEM-TIME"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.42. tsm_system_time</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="tsm-system-time.html#id-1.11.7.51.7">F.42.1. Examples</a></span></dt></dl></div><a id="id-1.11.7.51.2" class="indexterm"></a><p>
- The <code class="filename">tsm_system_time</code> module provides the table sampling method
- <code class="literal">SYSTEM_TIME</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 floating-point argument that
- is the maximum number of milliseconds to spend reading the table. This
- gives you direct control over how long the query takes, at the price that
- the size of the sample becomes hard to predict. The resulting sample will
- contain as many rows as could be read in the specified time, unless the
- whole table has been read first.
- </p><p>
- Like the built-in <code class="literal">SYSTEM</code> sampling
- method, <code class="literal">SYSTEM_TIME</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 selected.
- </p><p>
- <code class="literal">SYSTEM_TIME</code> does not support
- the <code class="literal">REPEATABLE</code> clause.
- </p><div class="sect2" id="id-1.11.7.51.7"><div class="titlepage"><div><div><h3 class="title">F.42.1. Examples</h3></div></div></div><p>
- Here is an example of selecting a sample of a table with
- <code class="literal">SYSTEM_TIME</code>. First install the extension:
- </p><pre class="programlisting">
- CREATE EXTENSION tsm_system_time;
- </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_TIME(1000);
- </pre><p>
- </p><p>
- This command will return as large a sample of <code class="structname">my_table</code> as
- it can read in 1 second (1000 milliseconds). Of course, if the whole
- table can be read in under 1 second, all its rows will be returned.
- </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tsm-system-rows.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="unaccent.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.41. tsm_system_rows </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> F.43. unaccent</td></tr></table></div></body></html>
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