CREATE FUNCTION -- Defines a new function
CREATE FUNCTION name ( [ ftype [, ...] ] )
RETURNS rtype
AS definition
LANGUAGE 'langname ' [ WITH ( attribute [, ...] ) ]
CREATE FUNCTION name ( [ ftype [, ...] ] )
RETURNS rtype
AS obj_file , link_symbol
LANGUAGE 'C' [ WITH ( attribute [, ...] ) ]
CREATE FUNCTION allows a Postgres user to register a function with a database. Subsequently, this user is considered the owner of the function.
Refer to the chapter in the PostgreSQL Programmer's Guide on the topic of extending Postgres via functions for further information on writing external functions.
Use DROP FUNCTION to remove 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.
The full SQL92 type syntax is allowed for input arguments and return value. However, some details of the type specification (e.g., the precision field for numeric types) are the responsibility of the underlying function implementation and are silently swallowed (e.g., not recognized or enforced) by the CREATE FUNCTION command.
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.
When overloading SQL functions with C-language functions, give each C-language instance of the function a distinct name, and use the alternative form of the AS clause in the CREATE FUNCTION syntax to ensure that overloaded SQL functions names are resolved to the correct dynamically linked objects.
A C function cannot return a set of values.
To create a simple SQL function:
CREATE FUNCTION one()
RETURNS int4 AS 'SELECT 1 AS RESULT'
LANGUAGE 'sql';
SELECT one() AS answer;
answer
--------
1
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 constraint.
CREATE FUNCTION ean_checkdigit(bpchar, bpchar)
RETURNS boolean
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))
);
This example creates a function that does type conversion between
the user-defined type complex, and the internal type point. The function
is implemented by a dynamically loaded object that was compiled from
C source. For Postgres to find a type conversion function automatically,
the sql function has to have the same name as the return type, and
overloading is unavoidable. The function name is overloaded by using
the second form of the AS clause in the SQL definition:
CREATE FUNCTION point(complex) RETURNS point
AS '/home/bernie/pgsql/lib/complex.so', 'complex_to_point'
LANGUAGE 'c';
The C declaration of the function is:
Point * complex_to_point (Complex *z)
{
Point *p;
p = (Point *) palloc(sizeof(Point));
p->x = z->x;
p->y = z->y;
return p;
}
CREATE FUNCTION is a Postgres language extension.
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 ] type [, ...] ] )
RETURNS rtype
LANGUAGE 'langname'
ESPECIFIC routine