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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 61. Index Access Method Interface Definition</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="tableam.html" title="Chapter 60. Table Access Method Interface Definition" /><link rel="next" href="index-api.html" title="61.1. Basic API Structure for Indexes" /></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 61. Index Access Method Interface Definition</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tableam.html" title="Chapter 60. Table Access Method Interface Definition">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 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="index-api.html" title="61.1. Basic API Structure for Indexes">Next</a></td></tr></table><hr></hr></div><div class="chapter" id="INDEXAM"><div class="titlepage"><div><div><h2 class="title">Chapter 61. Index Access Method Interface Definition</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="sect1"><a href="index-api.html">61.1. Basic API Structure for Indexes</a></span></dt><dt><span class="sect1"><a href="index-functions.html">61.2. Index Access Method Functions</a></span></dt><dt><span class="sect1"><a href="index-scanning.html">61.3. Index Scanning</a></span></dt><dt><span class="sect1"><a href="index-locking.html">61.4. Index Locking Considerations</a></span></dt><dt><span class="sect1"><a href="index-unique-checks.html">61.5. Index Uniqueness Checks</a></span></dt><dt><span class="sect1"><a href="index-cost-estimation.html">61.6. Index Cost Estimation Functions</a></span></dt></dl></div><a id="id-1.10.14.2" class="indexterm"></a><a id="id-1.10.14.3" class="indexterm"></a><p>
- This chapter defines the interface between the core
- <span class="productname">PostgreSQL</span> system and <em class="firstterm">index access
- methods</em>, which manage individual index types. The core system
- knows nothing about indexes beyond what is specified here, so it is
- possible to develop entirely new index types by writing add-on code.
- </p><p>
- All indexes in <span class="productname">PostgreSQL</span> are what are known
- technically as <em class="firstterm">secondary indexes</em>; that is, the index is
- physically separate from the table file that it describes. Each index
- is stored as its own physical <em class="firstterm">relation</em> and so is described
- by an entry in the <code class="structname">pg_class</code> catalog. The contents of an
- index are entirely under the control of its index access method. In
- practice, all index access methods divide indexes into standard-size
- pages so that they can use the regular storage manager and buffer manager
- to access the index contents. (All the existing index access methods
- furthermore use the standard page layout described in <a class="xref" href="storage-page-layout.html" title="68.6. Database Page Layout">Section 68.6</a>, and most use the same format for index
- tuple headers; but these decisions are not forced on an access method.)
- </p><p>
- An index is effectively a mapping from some data key values to
- <em class="firstterm">tuple identifiers</em>, or <acronym class="acronym">TIDs</acronym>, of row versions
- (tuples) in the index's parent table. A TID consists of a
- block number and an item number within that block (see <a class="xref" href="storage-page-layout.html" title="68.6. Database Page Layout">Section 68.6</a>). This is sufficient
- information to fetch a particular row version from the table.
- Indexes are not directly aware that under MVCC, there might be multiple
- extant versions of the same logical row; to an index, each tuple is
- an independent object that needs its own index entry. Thus, an
- update of a row always creates all-new index entries for the row, even if
- the key values did not change. (HOT tuples are an exception to this
- statement; but indexes do not deal with those, either.) Index entries for
- dead tuples are reclaimed (by vacuuming) when the dead tuples themselves
- are reclaimed.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tableam.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="internals.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="index-api.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 60. Table Access Method Interface Definition </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 61.1. Basic API Structure for Indexes</td></tr></table></div></body></html>
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