<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!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 Vsnapshot" /><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 11.5 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>
