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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>13.1. Introduction</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.html" title="Chapter 13. Concurrency Control" /><link rel="next" href="transaction-iso.html" title="13.2. Transaction Isolation" /></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">13.1. Introduction</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="mvcc.html" title="Chapter 13. Concurrency Control">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="transaction-iso.html" title="13.2. Transaction Isolation">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="MVCC-INTRO"><div class="titlepage"><div><div><h2 class="title" style="clear: both">13.1. Introduction</h2></div></div></div><a id="id-1.5.12.4.2" class="indexterm"></a><a id="id-1.5.12.4.3" class="indexterm"></a><a id="id-1.5.12.4.4" class="indexterm"></a><a id="id-1.5.12.4.5" class="indexterm"></a><p>
<span class="productname">PostgreSQL</span> provides a rich set of tools
for developers to manage concurrent access to data. Internally,
data consistency is maintained by using a multiversion
model (Multiversion Concurrency Control, <acronym class="acronym">MVCC</acronym>).
This means that each SQL statement sees
a snapshot of data (a <em class="firstterm">database version</em>)
as it was some
time ago, regardless of the current state of the underlying data.
This prevents statements from viewing inconsistent data produced
by concurrent transactions performing updates on the same
data rows, providing <em class="firstterm">transaction isolation</em>
for each database session. <acronym class="acronym">MVCC</acronym>, by eschewing
the locking methodologies of traditional database systems,
minimizes lock contention in order to allow for reasonable
performance in multiuser environments.
</p><p>
The main advantage of using the <acronym class="acronym">MVCC</acronym> model of
concurrency control rather than locking is that in
<acronym class="acronym">MVCC</acronym> locks acquired for querying (reading) data
do not conflict with locks acquired for writing data, and so
reading never blocks writing and writing never blocks reading.
<span class="productname">PostgreSQL</span> maintains this guarantee
even when providing the strictest level of transaction
isolation through the use of an innovative <em class="firstterm">Serializable
Snapshot Isolation</em> (<acronym class="acronym">SSI</acronym>) level.
</p><p>
Table- and row-level locking facilities are also available in
<span class="productname">PostgreSQL</span> for applications which don't
generally need full transaction isolation and prefer to explicitly
manage particular points of conflict. However, proper
use of <acronym class="acronym">MVCC</acronym> will generally provide better
performance than locks. In addition, application-defined advisory
locks provide a mechanism for acquiring locks that are not tied
to a single transaction.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="mvcc.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="mvcc.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="transaction-iso.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 13. Concurrency Control </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 13.2. Transaction Isolation</td></tr></table></div></body></html>
