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CREATE FUNCTION

Name

CREATE FUNCTION — Defines a new function
CREATE FUNCTION name ( [ ftype [, ...] ] )
    RETURNS rtype
    AS definition
    LANGUAGE 'langname'
  

Inputs

name

The name of a function to create.

ftype

The data type of function arguments.

rtype

The return data type.

definition

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.

langname

may be 'C', 'sql', 'internal' or 'plname', where 'plname' is the name of a created procedural language. See CREATE LANGUAGE for details.

Outputs

CREATE

This is returned if the command completes successfully.

Description

CREATE FUNCTION allows a Postgres user to register a function with a database. Subsequently, this user is treated as the owner of the function.

Notes

Refer to the chapter on functions in the PostgreSQL Programmer's Guide for further information.

Use DROP FUNCTION to drop user-defined functions.

Postgres 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 INTERNAL and C-language functions, however.

Two INTERNAL 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 CREATE FUNCTION. If the AS clause is left empty then CREATE FUNCTION assumes the C name of the function is the same as the SQL name.

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.

Usage

To create a simple SQL function:

CREATE FUNCTION one() RETURNS int4
    AS 'SELECT 1 AS RESULT'
    LANGUAGE 'sql';
SELECT one() AS answer;

       answer 
    ------
    1
   
  

To create a C function, 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.

   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))
);
   
  

Bugs

A C function cannot return a set of values.

Compatibility

CREATE FUNCTION is a Postgres language extension.

SQL/PSM

Note: 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.

SQL/PSM CREATE FUNCTION has the following syntax:
CREATE FUNCTION name
    ( [ [ IN | OUT | INOUT ] etereable>eable> type [, ...] ] )
     RETURNS rtype
     LANGUAGE 'langname'
     ESPECIFIC routine
     SQL-statement
    
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