CREATE FUNCTIONSQL - Language StatementsCREATE FUNCTIONDefines a new function
CREATE FUNCTION name ( [ argtype [, ...] ] )
RETURNS rettype
AS 'definition'
LANGUAGE 'langname'
[ WITH ( attribute [, ...] ) ]
CREATE FUNCTION name ( [ argtype [, ...] ] )
RETURNS rettype
AS 'obj_file', 'link_symbol'
LANGUAGE 'langname'
[ WITH ( attribute [, ...] ) ]
DescriptionCREATE FUNCTION defines a new function.
Parametersname
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, or
opaque. Opaque indicates
that the function accepts arguments of a non-SQL type such as
char *.
rettype
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. 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
for details.
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
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
to know whether it is safe to
pre-evaluate a call of the function.
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 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
Postgres via functions
for further information on writing external functions.
Use DROP FUNCTION
to remove user-defined 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.
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.
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 command.
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. 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 '/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 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,
,
PostgreSQL Programmer's Guide