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   2 <!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 id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">51.4. The <span 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 18.0 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 /></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 <a href="#RULE-SYSTEM" class="id_link">#</a></h2></div></div></div><p>
   3     <span class="productname">PostgreSQL</span> supports a powerful
   4     <em class="firstterm">rule system</em> for the specification
   5     of <em class="firstterm">views</em> and ambiguous <em class="firstterm">view updates</em>.
   6     Originally the <span class="productname">PostgreSQL</span>
   7     rule system consisted of two implementations:
   8
   9     </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
  10        The first one worked using <em class="firstterm">row level</em> processing and was
  11        implemented deep in the <em class="firstterm">executor</em>. The rule system was
  12        called whenever an individual row had been accessed. This
  13        implementation was removed in 1995 when the last official release
  14        of the <span class="productname">Berkeley Postgres</span> project was
  15        transformed into <span class="productname">Postgres95</span>.
  16       </p></li><li class="listitem"><p>
  17        The second implementation of the rule system is a technique
  18        called <em class="firstterm">query rewriting</em>.
  19        The <em class="firstterm">rewrite system</em> is a module
  20        that exists between the <em class="firstterm">parser stage</em> and the
  21        <em class="firstterm">planner/optimizer</em>. This technique is still implemented.
  22       </p></li></ul></div><p>
  23    </p><p>
  24     The query rewriter is discussed in some detail in
  25     <a class="xref" href="rules.html" title="Chapter 39. The Rule System">Chapter 39</a>, so there is no need to cover it here.
  26     We will only point out that both the input and the output of the
  27     rewriter are query trees, that is, there is no change in the
  28     representation or level of semantic detail in the trees.  Rewriting
  29     can be thought of as a form of macro expansion.
  30    </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="parser-stage.html" title="51.3. The Parser Stage">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="overview.html" title="Chapter 51. Overview of PostgreSQL Internals">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="planner-optimizer.html" title="51.5. Planner/Optimizer">Next</a></td></tr><tr><td width="40%" align="left" valign="top">51.3. The Parser Stage </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 51.5. Planner/Optimizer</td></tr></table></div></body></html>