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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 Custom Scan Provider</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 V1.79.1" /><link rel="prev" href="tablesample-support-functions.html" title="57.1. Sampling Method Support Functions" /><link rel="next" href="custom-scan-path.html" title="58.1. Creating Custom Scan Paths" /></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 Custom Scan Provider</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tablesample-support-functions.html" title="57.1. Sampling Method Support Functions">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 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="custom-scan-path.html" title="58.1. Creating Custom Scan Paths">Next</a></td></tr></table><hr></hr></div><div class="chapter" id="CUSTOM-SCAN"><div class="titlepage"><div><div><h2 class="title">Chapter 58. Writing a Custom Scan Provider</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="sect1"><a href="custom-scan-path.html">58.1. Creating Custom Scan Paths</a></span></dt><dd><dl><dt><span class="sect2"><a href="custom-scan-path.html#CUSTOM-SCAN-PATH-CALLBACKS">58.1.1. Custom Scan Path Callbacks</a></span></dt></dl></dd><dt><span class="sect1"><a href="custom-scan-plan.html">58.2. Creating Custom Scan Plans</a></span></dt><dd><dl><dt><span class="sect2"><a href="custom-scan-plan.html#CUSTOM-SCAN-PLAN-CALLBACKS">58.2.1. Custom Scan Plan Callbacks</a></span></dt></dl></dd><dt><span class="sect1"><a href="custom-scan-execution.html">58.3. Executing Custom Scans</a></span></dt><dd><dl><dt><span class="sect2"><a href="custom-scan-execution.html#CUSTOM-SCAN-EXECUTION-CALLBACKS">58.3.1. Custom Scan Execution Callbacks</a></span></dt></dl></dd></dl></div><a id="id-1.10.11.2" class="indexterm"></a><p>
+  <span class="productname">PostgreSQL</span> supports a set of experimental facilities which
+  are intended to allow extension modules to add new scan types to the system.
+  Unlike a <a class="link" href="fdwhandler.html" title="Chapter 56. Writing a Foreign Data Wrapper">foreign data wrapper</a>, which is only
+  responsible for knowing how to scan its own foreign tables, a custom scan
+  provider can provide an alternative method of scanning any relation in the
+  system.  Typically, the motivation for writing a custom scan provider will
+  be to allow the use of some optimization not supported by the core
+  system, such as caching or some form of hardware acceleration.  This chapter
+  outlines how to write a new custom scan provider.
+ </p><p>
+  Implementing a new type of custom scan is a three-step process.  First,
+  during planning, it is necessary to generate access paths representing a
+  scan using the proposed strategy.  Second, if one of those access paths
+  is selected by the planner as the optimal strategy for scanning a
+  particular relation, the access path must be converted to a plan.
+  Finally, it must be possible to execute the plan and generate the same
+  results that would have been generated for any other access path targeting
+  the same relation.
+ </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="tablesample-support-functions.html" title="57.1. Sampling Method Support Functions">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="custom-scan-path.html" title="58.1. Creating Custom Scan Paths">Next</a></td></tr><tr><td width="40%" align="left" valign="top">57.1. Sampling Method Support Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 58.1. Creating Custom Scan Paths</td></tr></table></div></body></html>
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