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+<?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>69.2. Implementation</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="hash-intro.html" title="69.1. Overview" /><link rel="next" href="storage.html" title="Chapter 70. Database Physical Storage" /></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">69.2. Implementation</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="hash-intro.html" title="69.1. Overview">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="hash-index.html" title="Chapter 69. Hash Indexes">Up</a></td><th width="60%" align="center">Chapter 69. Hash Indexes</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="storage.html" title="Chapter 70. Database Physical Storage">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="HASH-IMPLEMENTATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">69.2. Implementation</h2></div></div></div><p>
+ There are four kinds of pages in a hash index: the meta page (page zero),
+ which contains statically allocated control information; primary bucket
+ pages; overflow pages; and bitmap pages, which keep track of overflow
+ pages that have been freed and are available for re-use. For addressing
+ purposes, bitmap pages are regarded as a subset of the overflow pages.
+ </p><p>
+ Both scanning the index and inserting tuples require locating the bucket
+ where a given tuple ought to be located. To do this, we need the bucket
+ count, highmask, and lowmask from the metapage; however, it's undesirable
+ for performance reasons to have to have to lock and pin the metapage for
+ every such operation. Instead, we retain a cached copy of the metapage
+ in each backend's relcache entry. This will produce the correct bucket
+ mapping as long as the target bucket hasn't been split since the last
+ cache refresh.
+ </p><p>
+ Primary bucket pages and overflow pages are allocated independently since
+ any given index might need more or fewer overflow pages relative to its
+ number of buckets. The hash code uses an interesting set of addressing
+ rules to support a variable number of overflow pages while not having to
+ move primary bucket pages around after they are created.
+ </p><p>
+ Each row in the table indexed is represented by a single index tuple in
+ the hash index. Hash index tuples are stored in bucket pages, and if
+ they exist, overflow pages. We speed up searches by keeping the index entries
+ in any one index page sorted by hash code, thus allowing binary search to be
+ used within an index page. Note however that there is *no* assumption about
+ the relative ordering of hash codes across different index pages of a bucket.
+ </p><p>
+ The bucket splitting algorithms to expand the hash index are too complex to
+ be worthy of mention here, though are described in more detail in
+ <code class="filename">src/backend/access/hash/README</code>.
+ The split algorithm is crash safe and can be restarted if not completed
+ successfully.
+ </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="hash-intro.html" title="69.1. Overview">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="hash-index.html" title="Chapter 69. Hash Indexes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="storage.html" title="Chapter 70. Database Physical Storage">Next</a></td></tr><tr><td width="40%" align="left" valign="top">69.1. Overview </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"> Chapter 70. Database Physical Storage</td></tr></table></div></body></html> \ No newline at end of file