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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>13.6. Locking and Indexes</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="mvcc-caveats.html" title="13.5. Caveats" /><link rel="next" href="performance-tips.html" title="Chapter 14. Performance Tips" /></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">13.6. Locking and Indexes</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="mvcc-caveats.html" title="13.5. Caveats">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="mvcc.html" title="Chapter 13. Concurrency Control">Up</a></td><th width="60%" align="center">Chapter 13. Concurrency Control</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="performance-tips.html" title="Chapter 14. Performance Tips">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LOCKING-INDEXES"><div class="titlepage"><div><div><h2 class="title" style="clear: both">13.6. Locking and Indexes</h2></div></div></div><a id="id-1.5.12.9.2" class="indexterm"></a><p>
Though <span class="productname">PostgreSQL</span>
provides nonblocking read/write access to table
data, nonblocking read/write access is not currently offered for every
index access method implemented
in <span class="productname">PostgreSQL</span>.
The various index types are handled as follows:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">
B-tree, <acronym class="acronym">GiST</acronym> and <acronym class="acronym">SP-GiST</acronym> indexes
</span></dt><dd><p>
Short-term share/exclusive page-level locks are used for
read/write access. Locks are released immediately after each
index row is fetched or inserted. These index types provide
the highest concurrency without deadlock conditions.
</p></dd><dt><span class="term">
Hash indexes
</span></dt><dd><p>
Share/exclusive hash-bucket-level locks are used for read/write
access. Locks are released after the whole bucket is processed.
Bucket-level locks provide better concurrency than index-level
ones, but deadlock is possible since the locks are held longer
than one index operation.
</p></dd><dt><span class="term">
<acronym class="acronym">GIN</acronym> indexes
</span></dt><dd><p>
Short-term share/exclusive page-level locks are used for
read/write access. Locks are released immediately after each
index row is fetched or inserted. But note that insertion of a
GIN-indexed value usually produces several index key insertions
per row, so GIN might do substantial work for a single value's
insertion.
</p></dd></dl></div><p>
</p><p>
Currently, B-tree indexes offer the best performance for concurrent
applications; since they also have more features than hash
indexes, they are the recommended index type for concurrent
applications that need to index scalar data. When dealing with
non-scalar data, B-trees are not useful, and GiST, SP-GiST or GIN
indexes should be used instead.
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