PostgreSQL User's Guide
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# CREATE FUNCTION

## Name

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 [, ...] ) ]

### Inputs

`name`

  The name of a function to create.

`ftype`

  The data type of function arguments. The input types may be base or complex types, or opaque. `opaque` indicates that the function accepts arguments of an invalid type such as char *.

`rtype`

  The return data type. The output type may be specified as a base type, complex type, `setof `type``, or `opaque`. The `setof` modifier indicates that the function will return a set of items, rather than a single item.

`attribute`

  An optional piece of information about the function, used for optimization. The only attribute currently supported is `iscachable`. `iscachable` indicates that the function always returns the same result when given the same input 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.

`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 which is the same as the name of the function in the C language source code.

`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 considered the owner of the function.

### Notes

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.

## 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

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.

    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))
    );

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 decalaration 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;
    }

## Compatibility

### SQL92

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 ] type [, ...] ] )
         RETURNS rtype
         LANGUAGE 'langname'
         ESPECIFIC routine
         SQL-statement

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