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CREATE
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 [create_language(5)]
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 [load(5)] 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(5), load(5), PostgreSQL Programmer's Guide
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Index for
Section FUNCTION |
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Alphabetical
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