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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 id="docContent" class="container-fluid col-10"><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 14.5 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 an 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.
+ </p></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="fdw-row-locking.html" title="57.5. Row Locking in Foreign Data Wrappers">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="internals.html" title="Part VII. Internals">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tablesample-support-functions.html" title="58.1. Sampling Method Support Functions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">57.5. Row Locking in Foreign Data Wrappers </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 58.1. Sampling Method Support Functions</td></tr></table></div></body></html>
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