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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>Chapter 58. Writing A Table Sampling Method</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="fdw-row-locking.html" title="57.5. Row Locking in Foreign Data Wrappers" /><link rel="next" href="tablesample-support-functions.html" title="58.1. Sampling Method Support Functions" /></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">Chapter 58. Writing A Table Sampling Method</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="fdw-row-locking.html" title="57.5. Row Locking in Foreign Data Wrappers">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="internals.html" title="Part VII. Internals">Up</a></td><th width="60%" align="center">Part VII. Internals</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="tablesample-support-functions.html" title="58.1. Sampling Method Support Functions">Next</a></td></tr></table><hr></hr></div><div class="chapter" id="TABLESAMPLE-METHOD"><div class="titlepage"><div><div><h2 class="title">Chapter 58. Writing A Table Sampling Method</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="sect1"><a href="tablesample-support-functions.html">58.1. Sampling Method Support Functions</a></span></dt></dl></div><a id="id-1.10.10.2" class="indexterm"></a><a id="id-1.10.10.3" class="indexterm"></a><p>
<span class="productname">PostgreSQL</span>'s implementation of the <code class="literal">TABLESAMPLE</code>
clause supports custom table sampling methods, in addition to
the <code class="literal">BERNOULLI</code> and <code class="literal">SYSTEM</code> methods that are required
by the SQL standard. The sampling method determines which rows of the
table will be selected when the <code class="literal">TABLESAMPLE</code> clause is used.
</p><p>
At the SQL level, a table sampling method is represented by a single SQL
function, typically implemented in C, having the signature
</p><pre class="programlisting">
method_name(internal) RETURNS tsm_handler
</pre><p>
The name of the function is the same method name appearing in the
<code class="literal">TABLESAMPLE</code> clause. The <code class="type">internal</code> argument is a dummy
(always having value zero) that simply serves to prevent this function from
being called directly from a SQL command.
The result of the function must be a palloc'd struct of
type <code class="type">TsmRoutine</code>, which contains pointers to support functions for
the sampling method. These support functions are plain C functions and
are not visible or callable at the SQL level. The support functions are
described in <a class="xref" href="tablesample-support-functions.html" title="58.1. Sampling Method Support Functions">Section 58.1</a>.
</p><p>
In addition to function pointers, the <code class="type">TsmRoutine</code> struct must
provide these additional fields:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">List *parameterTypes</code></span></dt><dd><p>
This is an OID list containing the data type OIDs of the parameter(s)
that will be accepted by the <code class="literal">TABLESAMPLE</code> clause when this
sampling method is used. For example, for the built-in methods, this
list contains a single item with value <code class="literal">FLOAT4OID</code>, which
represents the sampling percentage. Custom sampling methods can have
more or different parameters.
</p></dd><dt><span class="term"><code class="literal">bool repeatable_across_queries</code></span></dt><dd><p>
If <code class="literal">true</code>, the sampling method can deliver identical samples
across successive queries, if the same parameters
and <code class="literal">REPEATABLE</code> seed value are supplied each time and the
table contents have not changed. When this is <code class="literal">false</code>,
the <code class="literal">REPEATABLE</code> clause is not accepted for use with the
sampling method.
</p></dd><dt><span class="term"><code class="literal">bool repeatable_across_scans</code></span></dt><dd><p>
If <code class="literal">true</code>, the sampling method can deliver identical samples
across successive scans in the same query (assuming unchanging
parameters, seed value, and snapshot).
When this is <code class="literal">false</code>, the planner will not select plans that
would require scanning the sampled table more than once, since that
might result in inconsistent query output.
</p></dd></dl></div><p>
The <code class="type">TsmRoutine</code> struct type is declared
in <code class="filename">src/include/access/tsmapi.h</code>, which see for additional
details.
</p><p>
The table sampling methods included in the standard distribution are good
references when trying to write your own. Look into
the <code class="filename">src/backend/access/tablesample</code> subdirectory of the source
tree for the built-in sampling methods, and into the <code class="filename">contrib</code>
subdirectory for add-on methods.
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