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>34.3. User-Defined Functions</A
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>   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides four kinds of
   functions:

   <P
></P
></P><UL
><LI
><P
>      query language functions (functions written in
      <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>) (<A
HREF="xfunc-sql.html"
>Section 34.4</A
>)
     </P
></LI
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>      procedural language functions (functions written in, for
      example, <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> or <SPAN
CLASS="APPLICATION"
>PL/Tcl</SPAN
>)
      (<A
HREF="xfunc-pl.html"
>Section 34.7</A
>)
     </P
></LI
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>      internal functions (<A
HREF="xfunc-internal.html"
>Section 34.8</A
>)
     </P
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>      C-language functions (<A
HREF="xfunc-c.html"
>Section 34.9</A
>)
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  </P
><P
>   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.
  </P
><P
>   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.
  </P
><P
>   It's easiest to define <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>
   functions, so we'll start by discussing those.
   Most of the concepts presented for <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> functions
   will carry over to the other types of functions.
  </P
><P
>   Throughout this chapter, it can be useful to look at the reference
   page of the <A
HREF="sql-createfunction.html"
><I
>CREATE FUNCTION</I
></A
> command to
   understand the examples better.  Some examples from this chapter
   can be found in <TT
CLASS="FILENAME"
>funcs.sql</TT
> and
   <TT
CLASS="FILENAME"
>funcs.c</TT
> in the <TT
CLASS="FILENAME"
>src/tutorial</TT
>
   directory in the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> source
   distribution.
  </P
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>Query Language (<ACRONYM
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