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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 V1.79.1" /><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 12.4 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>
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