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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
>)
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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.
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>
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