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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.47. 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 Vsnapshot" /><link rel="prev" href="tsm-system-rows.html" title="F.46. tsm_system_rows" /><link rel="next" href="unaccent.html" title="F.48. unaccent" /></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.47. tsm_system_time</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tsm-system-rows.html" title="F.46. 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 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="unaccent.html" title="F.48. unaccent">Next</a></td></tr></table><hr /></div><div class="sect1" id="TSM-SYSTEM-TIME"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.47. tsm_system_time</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="tsm-system-time.html#id-1.11.7.56.8">F.47.1. Examples</a></span></dt></dl></div><a id="id-1.11.7.56.2" 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="link" href="sql-select.html" title="SELECT"><code class="command">SELECT</code></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><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-1.11.7.56.8"><div class="titlepage"><div><div><h3 class="title">F.47.1. 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" title="F.46. tsm_system_rows">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="unaccent.html" title="F.48. unaccent">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.46. tsm_system_rows </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> F.48. unaccent</td></tr></table></div></body></html>
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