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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>51.4. The PostgreSQL Rule System</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="parser-stage.html" title="51.3. The Parser Stage" /><link rel="next" href="planner-optimizer.html" title="51.5. Planner/Optimizer" /></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">51.4. The <span xmlns="http://www.w3.org/1999/xhtml" class="productname">PostgreSQL</span> Rule System</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="parser-stage.html" title="51.3. The Parser Stage">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="overview.html" title="Chapter 51. Overview of PostgreSQL Internals">Up</a></td><th width="60%" align="center">Chapter 51. Overview of PostgreSQL Internals</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="planner-optimizer.html" title="51.5. Planner/Optimizer">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="RULE-SYSTEM"><div class="titlepage"><div><div><h2 class="title" style="clear: both">51.4. The <span class="productname">PostgreSQL</span> Rule System</h2></div></div></div><p>
<span class="productname">PostgreSQL</span> supports a powerful
<em class="firstterm">rule system</em> for the specification
of <em class="firstterm">views</em> and ambiguous <em class="firstterm">view updates</em>.
Originally the <span class="productname">PostgreSQL</span>
rule system consisted of two implementations:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
The first one worked using <em class="firstterm">row level</em> processing and was
implemented deep in the <em class="firstterm">executor</em>. The rule system was
called whenever an individual row had been accessed. This
implementation was removed in 1995 when the last official release
of the <span class="productname">Berkeley Postgres</span> project was
transformed into <span class="productname">Postgres95</span>.
</p></li><li class="listitem"><p>
The second implementation of the rule system is a technique
called <em class="firstterm">query rewriting</em>.
The <em class="firstterm">rewrite system</em> is a module
that exists between the <em class="firstterm">parser stage</em> and the
<em class="firstterm">planner/optimizer</em>. This technique is still implemented.
</p></li></ul></div><p>
</p><p>
The query rewriter is discussed in some detail in
<a class="xref" href="rules.html" title="Chapter 41. The Rule System">Chapter 41</a>, so there is no need to cover it here.
We will only point out that both the input and the output of the
rewriter are query trees, that is, there is no change in the
representation or level of semantic detail in the trees. Rewriting
can be thought of as a form of macro expansion.
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