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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 — the SYSTEM_TIME sampling method for TABLESAMPLE</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 — the SYSTEM_ROWS sampling method for TABLESAMPLE" /><link rel="next" href="unaccent.html" title="F.48. unaccent — a text search dictionary which removes diacritics" /></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 —
the <code class="literal">SYSTEM_TIME</code> sampling method for <code class="literal">TABLESAMPLE</code></th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tsm-system-rows.html" title="F.46. tsm_system_rows — the SYSTEM_ROWS sampling method for TABLESAMPLE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules and Extensions</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="unaccent.html" title="F.48. unaccent — a text search dictionary which removes diacritics">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 —
the <code class="literal">SYSTEM_TIME</code> sampling method for <code class="literal">TABLESAMPLE</code> <a href="#TSM-SYSTEM-TIME" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="tsm-system-time.html#TSM-SYSTEM-TIME-EXAMPLES">F.47.1. Examples</a></span></dt></dl></div><a id="id-1.11.7.57.2" 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="TSM-SYSTEM-TIME-EXAMPLES"><div class="titlepage"><div><div><h3 class="title">F.47.1. Examples <a href="#TSM-SYSTEM-TIME-EXAMPLES" class="id_link">#</a></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.
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