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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >How Extensibility Works</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Extending SQL" HREF="extend.html"><LINK REL="PREVIOUS" TITLE="Extending SQL" HREF="extend.html"><LINK REL="NEXT" TITLE="The PostgreSQL Type System" HREF="extend-type-system.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Extending SQL" HREF="extend.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="extend.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 35. Extending <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM ></TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="The PostgreSQL Type System" HREF="extend-type-system.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="EXTEND-HOW" >35.1. How Extensibility Works</A ></H1 ><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" >"on the fly"</SPAN > makes <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > uniquely suited for rapid prototyping of new applications and storage structures. </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="extend.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="extend-type-system.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Extending <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="extend.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >The <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > Type System</TD ></TR ></TABLE ></DIV ></BODY ></HTML >