PostgreSQL 7.2 Documentation
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# CREATE FUNCTION

## Name

CREATE FUNCTION -- define a new function

## Synopsis

    CREATE [ OR REPLACE ] FUNCTION name ( [ argtype [, ...] ] )
        RETURNS rettype
        AS 'definition'
        LANGUAGE langname
        [ WITH ( attribute [, ...] ) ]
    CREATE [ OR REPLACE ] FUNCTION name ( [ argtype [, ...] ] )
        RETURNS rettype
        AS 'obj_file', 'link_symbol'
        LANGUAGE langname
        [ WITH ( attribute [, ...] ) ]

## Description

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

Parameters

`name`

  The name of a function to create. The name need not be unique, because functions may be overloaded, but functions with the same name must have different argument types.

`argtype`

  The data type(s) of the function's arguments, if any. The input types may be base or complex types, `opaque`, or the same as the type of an existing column. `Opaque` indicates that the function accepts arguments of a non-SQL type such as `char *`. The type of a column is indicated using `tablename`.`columnname``%TYPE`; using this can sometimes help make a function independent from changes to the definition of a table.

`rettype`

  The return data type. The output type may be specified as a base type, complex type, `setof` type, `opaque`, or the same as the type of an existing column. The `setof` modifier indicates that the function will return a set of items, rather than a single item. Functions with a declared return type of `opaque` do not return a value. These cannot be called directly; trigger functions make use of this feature.

`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.

`obj_file`, `link_symbol`

  This form of the `AS` clause is used for dynamically linked C language functions when the function name in the C language source code is not the same as the name of the SQL function. The string `obj_file` is the name of the file containing the dynamically loadable object, and `link_symbol` is the object's link symbol, that is, the name of the function in the C language source code.

`langname`

  May be `SQL`, `C`, `internal`, or `plname`, where `plname` is the name of a created procedural language. See CREATE LANGUAGE for details. For backward compatibility, the name may be enclosed by single quotes.

`attribute`

  An optional piece of information about the function, used for optimization. See below for details.

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

The following attributes may appear in the WITH clause:

iscachable

  `Iscachable` indicates that the function always returns the same result when given the same argument values (i.e., it does not do database lookups or otherwise use information not directly present in its parameter list). The optimizer uses `iscachable` to know whether it is safe to pre-evaluate a call of the function.

isstrict

  `isstrict` indicates that the function always returns NULL whenever any of its arguments are NULL. If this attribute is specified, the function is not executed when there are NULL arguments; instead a NULL result is assumed automatically. When `isstrict` is not specified, the function will be called for NULL inputs. It is then the function author's responsibility to check for NULLs if necessary and respond appropriately.

## 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.

## 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))
    );

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 (we illustrate the now-deprecated alternative of specifying the exact pathname 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;

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;
    }

## Compatibility

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

## See Also

DROP FUNCTION , LOAD, PostgreSQL Programmer's Guide

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