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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>38.1. How Extensibility Works</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="extend.html" title="Chapter 38. Extending SQL" /><link rel="next" href="extend-type-system.html" title="38.2. The PostgreSQL Type System" /></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">38.1. How Extensibility Works</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="extend.html" title="Chapter 38. Extending SQL">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 38. Extending <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym></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="extend-type-system.html" title="38.2. The PostgreSQL Type System">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="EXTEND-HOW"><div class="titlepage"><div><div><h2 class="title" style="clear: both">38.1. How Extensibility Works</h2></div></div></div><p>
<span class="productname">PostgreSQL</span> is extensible because its operation is
catalog-driven. If you are familiar with standard
relational database systems, you know that they store information
about databases, tables, columns, etc., in what are
commonly known as system catalogs. (Some systems call
this the data dictionary.) The catalogs appear to the
user as tables like any other, but the <acronym class="acronym">DBMS</acronym> stores
its internal bookkeeping in them. One key difference
between <span class="productname">PostgreSQL</span> and standard relational database systems is
that <span class="productname">PostgreSQL</span> stores much more information in its
catalogs: not only information about tables and columns,
but also information about data types, functions, access
methods, and so on. These tables can be modified by
the user, and since <span class="productname">PostgreSQL</span> bases its operation
on these tables, this means that <span class="productname">PostgreSQL</span> can be
extended by users. By comparison, conventional
database systems can only be extended by changing hardcoded
procedures in the source code or by loading modules
specially written by the <acronym class="acronym">DBMS</acronym> vendor.
</p><p>
The <span class="productname">PostgreSQL</span> server can moreover
incorporate user-written code into itself through dynamic loading.
That is, the user can specify an object code file (e.g., a shared
library) that implements a new type or function, and
<span class="productname">PostgreSQL</span> will load it as required.
Code written in <acronym class="acronym">SQL</acronym> is even more trivial to add
to the server. This ability to modify its operation <span class="quote">“<span class="quote">on the
fly</span>”</span> makes <span class="productname">PostgreSQL</span> uniquely
suited for rapid prototyping of new applications and storage
structures.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="extend.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extend.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="extend-type-system.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 38. Extending <acronym class="acronym">SQL</acronym> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 38.2. The <span class="productname">PostgreSQL</span> Type System</td></tr></table></div></body></html>
