<!-- doc/src/sgml/extend.sgml -->

 <chapter id="extend">
  <title>Extending <acronym>SQL</acronym></title>

  <indexterm zone="extend">
   <primary>extending SQL</primary>
  </indexterm>

  <para>
   In  the  sections  that follow, we will discuss how you
   can extend the <productname>PostgreSQL</productname>
   <acronym>SQL</acronym> query language by adding:

   <itemizedlist spacing="compact" mark="bullet">
    <listitem>
     <para>
      functions (starting in <xref linkend="xfunc"/>)
     </para>
    </listitem>
    <listitem>
     <para>
      aggregates (starting in <xref linkend="xaggr"/>)
     </para>
    </listitem>
    <listitem>
     <para>
      data types (starting in <xref linkend="xtypes"/>)
     </para>
    </listitem>
    <listitem>
     <para>
      operators (starting in <xref linkend="xoper"/>)
     </para>
    </listitem>
    <listitem>
     <para>
      operator classes for indexes (starting in <xref linkend="xindex"/>)
     </para>
    </listitem>
    <listitem>
     <para>
      packages of related objects (starting in <xref linkend="extend-extensions"/>)
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <sect1 id="extend-how">
   <title>How Extensibility Works</title>

   <para>
    <productname>PostgreSQL</productname> 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>DBMS</acronym>  stores
    its  internal  bookkeeping in them.  One key difference
    between <productname>PostgreSQL</productname> and  standard  relational database systems  is
    that <productname>PostgreSQL</productname> 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 <productname>PostgreSQL</productname> bases its operation
    on these tables, this means that <productname>PostgreSQL</productname> 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>DBMS</acronym> vendor.
   </para>

   <para>
    The <productname>PostgreSQL</productname> 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
    <productname>PostgreSQL</productname> will load it as required.
    Code written in <acronym>SQL</acronym> is even more trivial to add
    to the server.  This ability to modify its operation <quote>on the
    fly</quote> makes <productname>PostgreSQL</productname> uniquely
    suited for rapid prototyping of new applications and storage
    structures.
   </para>
  </sect1>

  <sect1 id="extend-type-system">
   <title>The <productname>PostgreSQL</productname> Type System</title>

   <indexterm zone="extend-type-system">
    <primary>base type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>base</secondary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>composite type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>composite</secondary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>container type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>container</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname>