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

 <sect1 id="xfunc">
  <title>User-Defined Functions</title>

  <indexterm zone="xfunc">
   <primary>function</primary>
   <secondary>user-defined</secondary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> provides four kinds of
   functions:

   <itemizedlist>
    <listitem>
     <para>
      query language functions (functions written in
      <acronym>SQL</acronym>) (<xref linkend="xfunc-sql"/>)
     </para>
    </listitem>
    <listitem>
     <para>
      procedural language functions (functions written in, for
      example, <application>PL/pgSQL</application> or <application>PL/Tcl</application>)
      (<xref linkend="xfunc-pl"/>)
     </para>
    </listitem>
    <listitem>
     <para>
      internal functions (<xref linkend="xfunc-internal"/>)
     </para>
    </listitem>
    <listitem>
     <para>
      C-language functions (<xref linkend="xfunc-c"/>)
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   Every kind
   of  function  can take base types, composite types, or
   combinations of these as arguments (parameters). In addition,
   every kind of function can return a base type or
   a composite type.  Functions can also be defined to return
   sets of base or composite values.
  </para>

  <para>
   Many kinds of functions can take or return certain pseudo-types
   (such as polymorphic types), but the available facilities vary.
   Consult the description of each kind of function for more details.
  </para>

  <para>
   It's easiest to define <acronym>SQL</acronym>
   functions, so we'll start by discussing those.
   Most of the concepts presented for <acronym>SQL</acronym> functions
   will carry over to the other types of functions.
  </para>

  <para>
   Throughout this chapter, it can be useful to look at the reference
   page of the <link linkend="sql-createfunction"><command>CREATE
   FUNCTION</command></link> command to
   understand the examples better.  Some examples from this chapter
   can be found in <filename>funcs.sql</filename> and
   <filename>funcs.c</filename> in the <filename>src/tutorial</filename>
   directory in the <productname>PostgreSQL</productname> source
   distribution.
  </para>
  </sect1>

  <sect1 id="xproc">
   <title>User-Defined Procedures</title>

  <indexterm zone="xproc">
   <primary>procedure</primary>
   <secondary>user-defined</secondary>
  </indexterm>

   <para>
    A procedure is a database object similar to a function.
    The key differences are:

    <itemizedlist>
     <listitem>
      <para>
       Procedures are defined with
       the <link linkend="sql-createprocedure"><command>CREATE
       PROCEDURE</command></link> command, not <command>CREATE
       FUNCTION</command>.
      </para>
     </listitem>
     <listitem>
      <para>
       Procedures do not return a function value; hence <command>CREATE
       PROCEDURE</command> lacks a <literal>RETURNS</literal> clause.
       However, procedures can instead return data to their callers via
       output parameters.
      </para>
     </listitem>
     <listitem>
      <para>
       While a function is called as part of a query or DML command, a
       procedure is called in isolation using
       the <link linkend="sql-call"><command>CALL</command></link> command.
      </para>
     </listitem>
     <listitem>
      <para>
       A procedure can commit or roll back transactions during its
       execution (then automatically beginning a new transaction), so long
       as the invoking <command>CALL</command> command is not part of an
       explicit transaction block.  A function cannot do that.
      </para>
     </listitem>
     <listitem>
      <para>
       Certain function attributes, such as strictness, don't apply to
       procedures.  Those attributes control how the function is
       used in a query, which isn't relevant to procedures.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The explanations in the