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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_function.sgml,v 1.9 1999/07/22 15:09:07 thomas Exp $
Postgres documentation
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<refentry id="SQL-CREATEFUNCTION">
<refmeta>
<refentrytitle id="sql-createfunction-title">
</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
</refnamediv>
<date>1999-07-20</date>
CREATE FUNCTION <replaceable class="parameter">name</replaceable> ( [ <replaceable class="parameter">ftype</replaceable> [, ...] ] )
RETURNS <replaceable class="parameter">rtype</replaceable>
AS <replaceable class="parameter">definition</replaceable>
LANGUAGE '<replaceable class="parameter">langname</replaceable>'
<refsect2 id="R2-SQL-CREATEFUNCTION-1">
<refsect2info>
<date>1998-09-09</date>
</refsect2info>
<title>
<variablelist>
<varlistentry>
<term><replaceable class="parameter">name</replaceable></term>
<listitem>
<para>
The name of a function to create.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">ftype</replaceable></term>
<listitem>
<para>
The data type of function arguments.
The input types may be base or complex types, or
<firstterm>opaque</firstterm>.
<literal>opaque</literal> indicates that the function
accepts arguments of an invalid type such as <type>char *</type>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">rtype</replaceable></term>
<listitem>
<para>
The return data type.
The output type may be specified as a base type, complex type,
<literal>setof <replaceable class="parameter">type</replaceable></literal>,
or <literal>opaque</literal>.
The <literal>setof</literal>
modifier indicates that the function will return a set of items,
rather than a single item.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">definition</replaceable></term>
A string defining the function; the meaning depends on the language.
It may be an internal function name, the path to an object file,
an SQL query, or text in a procedural language.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">langname</replaceable></term>
may be '<literal>C</literal>', '<literal>sql</literal>',
'<literal>internal</literal>'
or '<replaceable class="parameter">plname</replaceable>',
where '<replaceable class="parameter">plname</replaceable>'
is the name of a created procedural
language. See
<xref linkend="sql-createlanguage-title" endterm="sql-createlanguage-title">
for details.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<refsect2 id="R2-SQL-CREATEFUNCTION-2">
<refsect2info>
<date>1998-09-09</date>
</refsect2info>
<title>
<term><computeroutput>
CREATE
</computeroutput></term>
This is returned if the command completes successfully.
</para>
</listitem>
</varlistentry>
</para>
<refsect1 id="R1-SQL-CREATEFUNCTION-1">
<refsect1info>
<date>1998-09-09</date>
</refsect1info>
<title>
<command>CREATE FUNCTION</command> allows a
<productname>Postgres</productname> user
to register a function
with a database. Subsequently, this user is treated as the
<refsect2 id="R2-SQL-CREATEFUNCTION-3">
<refsect2info>
<date>1998-09-09</date>
</refsect2info>
<title>
Refer to the chapter in
the <citetitle>PostgreSQL Programmer's Guide</citetitle>
on extending
<productname>Postgres</productname> via functions
for further information on writing external functions.
Use <command>DROP FUNCTION</command>
to drop user-defined functions.
<para>
<productname>Postgres</productname> allows function "overloading";
that is, the same name can be used for several different functions
so long as they have distinct argument types. This facility must be
used with caution for <literal>internal</literal>
and C-language functions, however.
</para>
<para>
Two <literal>internal</literal>
functions cannot have the same C name without causing
errors at link time. To get around that, give them different C names
(for example, use the argument types as part of the C names), then
specify those names in the AS clause of <command>CREATE FUNCTION</command>.
If the AS clause is left empty then <command>CREATE FUNCTION</command>
assumes the C name of the function is the same as the SQL name.
</para>
<para>
For dynamically-loaded C functions, the SQL name of the function must
be the same as the C function name, because the AS clause is used to
give the path name of the object file containing the C code. In this
situation it is best not to try to overload SQL function names. It
might work to load a C function that has the same C name as an internal
function or another dynamically-loaded function --- or it might not.
On some platforms the dynamic loader may botch the load in interesting
ways if there is a conflict of C function names. So, even if it works
for you today, you might regret overloading names later when you try
to run the code somewhere else.
</para>
<para>
A C function cannot return a set of values.
</para>
</refsect1>
<refsect1 id="R1-SQL-CREATEFUNCTION-2">
<title>
CREATE FUNCTION one() RETURNS int4
AS 'SELECT 1 AS RESULT'
LANGUAGE 'sql';
SELECT one() AS answer;
<computeroutput>
answer
------
1
</computeroutput>
This example creates a C function by calling a routine from a user-created
shared library. This particular routine calculates a check
digit and returns TRUE if the check digit in the function parameters
is correct. It is intended for use in a CHECK contraint.
</para>
<programlisting>
<userinput>
CREATE FUNCTION ean_checkdigit(bpchar, bpchar) RETURNS bool
AS '/usr1/proj/bray/sql/funcs.so' LANGUAGE 'c';
CREATE TABLE product (
id char(8) PRIMARY KEY,
eanprefix char(8) CHECK (eanprefix ~ '[0-9]{2}-[0-9]{5}')
REFERENCES brandname(ean_prefix),
eancode char(6) CHECK (eancode ~ '[0-9]{6}'),
CONSTRAINT ean CHECK (ean_checkdigit(eanprefix, eancode))
<refsect1 id="R1-SQL-CREATEFUNCTION-4">
<title>
<refsect2info>
<date>1998-04-15</date>
</refsect2info>
<title>
SQL92
</title>
<para>
<command>CREATE FUNCTION</command> is
a <productname>Postgres</productname> language extension.
</para>
</refsect2>
<refsect2 id="R2-SQL-CREATEFUNCTION-5">
<refsect2info>
<date>1998-09-09</date>
</refsect2info>
<title>
PSM stands for Persistent Stored Modules. It is a procedural
language and it was originally hoped that PSM would be ratified
as an official standard by late 1996. As of mid-1998, this
has not yet happened, but it is hoped that PSM will
eventually become a standard.
</para>
SQL/PSM <command>CREATE FUNCTION</command> has the following syntax:
<synopsis>
CREATE FUNCTION <replaceable class="parameter">name</replaceable>
( [ [ IN | OUT | INOUT ] <replaceable class="parameter">eter</replaceable>eable>eable> <replaceable
class="parameter">type</replaceable> [, ...] ] )
RETURNS <replaceable class="parameter">rtype</replaceable>
LANGUAGE '<replaceable class="parameter">langname</replaceable>'
ESPECIFIC <replaceable class="parameter">routine</replaceable>
<replaceable class="parameter">SQL-statement</replaceable>
</para>
</refsect2>
</refsect1>
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