CREATE FUNCTION

Description

CREATE FUNCTION defines a new function. CREATE OR REPLACE FUNCTION will either create a new function, or replace an existing definition.

The user that creates the function becomes the owner of the function.

Notes

Refer to the chapter in the PostgreSQL Programmer's Guide on the topic of extending PostgreSQL via functions for further information on writing external functions.

The full SQL 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 (i.e., not recognized or enforced) by the CREATE FUNCTION command.

PostgreSQL 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.

Similarly, when overloading SQL function names with multiple C-language functions, give each C-language instance of the function a distinct name, then use the alternative form of the AS clause in the CREATE FUNCTION syntax to select the appropriate C-language implementation of each overloaded SQL function.

When repeated CREATE FUNCTION calls refer to the same object file, the file is only loaded once. To unload and reload the file (perhaps during development), use the LOAD command.

Use DROP FUNCTION to remove user-defined functions.

To update the definition of an existing function, use CREATE OR REPLACE FUNCTION. Note that it is not possible to change the name or argument types of a function this way (if you tried, you'd just be creating a new, distinct function). Also, CREATE OR REPLACE FUNCTION will not let you change the return type of an existing function. To do that, you must drop and re-create the function.

If you drop and then re-create a function, the new function is not the same entity as the old; you will break existing rules, views, triggers, etc that referred to the old function. Use CREATE OR REPLACE FUNCTION to change a function definition without breaking objects that refer to the function.

To be able to define a function, the user must have the USAGE privilege on the language.

By default, only the owner (creator) of the function has the right to execute it. Other users must be granted the EXECUTE privilege on the function to be able to use it.

Examples

To create a simple SQL function:

CREATE FUNCTION one() RETURNS integer
    AS 'SELECT 1 AS RESULT;'
    LANGUAGE SQL;

SELECT one() AS answer;
 answer 
--------
      1

The next example creates a C function by calling a routine from a user-created shared library named funcs.so (the extension may vary across platforms). The shared library file is sought in the server's dynamic library search path. 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(char, char) RETURNS boolean
    AS 'funcs' 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))
);

The next example creates a function that does type conversion from the user-defined type complex to the built-in type point. The function is implemented by a dynamically loaded object that was compiled from C source (we illustrate the now-deprecated alternative of specifying the absolute file name to the shared object file). For PostgreSQL to find a type conversion function automatically, the SQL function has to have the same name as the return type, and so 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 STRICT;

The C declaration of the function could be:

Point * complex_to_point (Complex *z)
{
        Point *p;

        p = (Point *) palloc(sizeof(Point));
        p->x = z->x;
        p->y = z->y;
                
        return p;
}

Note that the function is marked "strict"; this allows us to skip checking for NULL input in the function body.

Writing SECURITY DEFINER Functions Safely

Because a SECURITY DEFINER function is executed with the privileges of the user that created it, care is needed to ensure that the function cannot be misused. For security, search_path should be set to exclude any schemas writable by untrusted users. This prevents malicious users from creating objects that mask objects used by the function. Particularly important in this regard is the temporary-table schema, which is searched first by default, and is normally writable by anyone. A secure arrangement can be had by forcing the temporary schema to be searched last. To do this, write pg_temp as the last entry in search_path. This function illustrates safe usage:

CREATE FUNCTION check_password(TEXT, TEXT)
RETURNS BOOLEAN AS '
DECLARE passed BOOLEAN;
        old_path TEXT;
BEGIN
        -- Save old search_path; notice we must qualify current_setting
        -- to ensure we invoke the right function
        old_path := pg_catalog.current_setting(''search_path'');

        -- Set a secure search_path: trusted schemas, then ''pg_temp''.
        -- We set is_local = true so that the old value will be restored
        -- in event of an error before we reach the function end.
        PERFORM pg_catalog.set_config(''search_path'', ''admin, pg_temp'', true);

        -- Do whatever secure work we came for.
        SELECT  (pwd = $2) INTO passed
        FROM    pwds
        WHERE   username = $1;

        -- Restore caller''s search_path
        PERFORM pg_catalog.set_config(''search_path'', old_path, true);

        RETURN passed;
END;
' LANGUAGE plpgsql SECURITY DEFINER;

Compatibility

A CREATE FUNCTION command is defined in SQL99. The PostgreSQL version is similar but not fully compatible. The attributes are not portable, neither are the different available languages.

See Also

DROP FUNCTION, GRANT, LOAD, REVOKE, createlang, PostgreSQL Programmer's Guide