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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 id="docContent" class="container-fluid col-10"><div 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 15.7 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 /></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" title="Chapter 13. Concurrency Control">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="mvcc.html" title="Chapter 13. Concurrency Control">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="transaction-iso.html" title="13.2. Transaction Isolation">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" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 13.2. Transaction Isolation</td></tr></table></div></body></html>