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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-13 13:44:03 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-13 13:44:03 +0000 |
| commit | 293913568e6a7a86fd1479e1cff8e2ecb58d6568 (patch) | |
| tree | fc3b469a3ec5ab71b36ea97cc7aaddb838423a0c /doc/src/sgml/html/tablesample-method.html | |
| parent | Initial commit. (diff) | |
| download | postgresql-16-293913568e6a7a86fd1479e1cff8e2ecb58d6568.tar.xz postgresql-16-293913568e6a7a86fd1479e1cff8e2ecb58d6568.zip | |
Adding upstream version 16.2.upstream/16.2
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'doc/src/sgml/html/tablesample-method.html')
| -rw-r--r-- | doc/src/sgml/html/tablesample-method.html | 57 |
1 files changed, 57 insertions, 0 deletions
diff --git a/doc/src/sgml/html/tablesample-method.html b/doc/src/sgml/html/tablesample-method.html new file mode 100644 index 0000000..c67a6d6 --- /dev/null +++ b/doc/src/sgml/html/tablesample-method.html @@ -0,0 +1,57 @@ +<?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>Chapter 60. 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="59.5. Row Locking in Foreign Data Wrappers" /><link rel="next" href="tablesample-support-functions.html" title="60.1. Sampling Method Support Functions" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">Chapter 60. Writing a Table Sampling Method</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="fdw-row-locking.html" title="59.5. 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 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="tablesample-support-functions.html" title="60.1. Sampling Method Support Functions">Next</a></td></tr></table><hr /></div><div class="chapter" id="TABLESAMPLE-METHOD"><div class="titlepage"><div><div><h2 class="title">Chapter 60. 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">60.1. Sampling Method Support Functions</a></span></dt></dl></div><a id="id-1.10.11.2" class="indexterm"></a><a id="id-1.10.11.3" 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="60.1. Sampling Method Support Functions">Section 60.1</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 class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="fdw-row-locking.html" title="59.5. 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="60.1. Sampling Method Support Functions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">59.5. Row Locking in Foreign Data Wrappers </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 60.1. Sampling Method Support Functions</td></tr></table></div></body></html>
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