CREATE FUNCTIOSQL)- Language Statements (2002-1CREATE FUNCTION(l)
NAME
CREATE FUNCTION - define a new function
SYNOPSIS
CREATE [ OR REPLACE ] FUNCTION name ( [ argtype [, ...] ] )
RETURNS rettype
{ LANGUAGE langname
| IMMUTABLE | STABLE | VOLATILE
| CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
| [EXTERNAL] SECURITY INVOKER | [EXTERNAL] SECURITY DEFINER
| AS 'definition'
| AS 'obj_file', 'link_symbol'
} ...
[ WITH ( attribute [, ...] ) ]
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. "PARAMETERS"
name The name of a function to create. If a schema name is
included, then the function is created in the specified
schema. Otherwise it is created in the current schema
(the one at the front of the search path; see
CURRENT_SCHEMA()). The name of the new function must
not match any existing function with the same argument
types in the same schema. However, functions of
different argument types may share a name (this is
called overloading).
argtype
The data type(s) of the function's arguments, if any.
The input types may be base, complex, or domain types,
or the same as the type of an existing column. The
type of a column is referenced by writing
tablename.columnname%TYPE; using this can sometimes
help make a function independent from changes to the
definition of a table. Depending on the implementation
language it may also be allowed to specify ``pseudo-
types'' such as cstring. Pseudo-types indicate that
the actual argument type is either incompletely
specified, or outside the set of ordinary SQL data
types.
rettype
The return data type. The return type may be specified
as a base, complex, or domain type, or the same as the
type of an existing column. Depending on the
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CREATE FUNCTIOSQL)- Language Statements (2002-1CREATE FUNCTION(l)
implementation language it may also be allowed to
specify ``pseudo-types'' such as cstring. The setof
modifier indicates that the function will return a set
of items, rather than a single item.
langname
The name of the language that the function is
implemented in. May be SQL, C, internal, or the name
of a user-defined procedural language. (See also
createlang [createlang(1)].) For backward
compatibility, the name may be enclosed by single
quotes.
IMMUTABLE
STABLE
VOLATILE
These attributes inform the system whether it is safe
to replace multiple evaluations of the function with a
single evaluation, for run-time optimization. At most
one choice should be specified. If none of these
appear, VOLATILE is the default assumption.
IMMUTABLE indicates that the function always returns
the same result when given the same argument values;
that is, it does not do database lookups or otherwise
use information not directly present in its parameter
list. If this option is given, any call of the function
with all-constant arguments can be immediately replaced
with the function value.
STABLE indicates that within a single table scan the
function will consistently return the same result for
the same argument values, but that its result could
change across SQL statements. This is the appropriate
selection for functions whose results depend on
database lookups, parameter variables (such as the
current time zone), etc. Also note that the
CURRENT_TIMESTAMP family of functions qualify as
stable, since their values do not change within a
transaction.
VOLATILE indicates that the function value can change
even within a single table scan, so no optimizations
can be made. Relatively few database functions are
volatile in this sense; some examples are random(),
currval(), timeofday(). Note that any function that has
side-effects must be classified volatile, even if its
result is quite predictable, to prevent calls from
being optimized away; an example is setval().
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CREATE FUNCTIOSQL)- Language Statements (2002-1CREATE FUNCTION(l)
CALLED ON NULL INPUT
RETURNS NULL ON NULL INPUT
STRICT
CALLED ON NULL INPUT (the default) indicates that the
function will be called normally when some of its
arguments are null. It is then the function author's
responsibility to check for null values if necessary
and respond appropriately.
RETURNS NULL ON NULL INPUT or STRICT indicates that the
function always returns NULL whenever any of its
arguments are NULL. If this parameter is specified, the
function is not executed when there are NULL arguments;
instead a NULL result is assumed automatically.
[EXTERNAL] SECURITY INVOKER
[EXTERNAL] SECURITY DEFINER
SECURITY INVOKER indicates that the function is to be
executed with the privileges of the user that calls it.
That is the default. SECURITY DEFINER specifies that
the function is to be executed with the privileges of
the user that created it.
The key word EXTERNAL is present for SQL compatibility
but is optional since, unlike in SQL, this feature does
not only apply to external functions.
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.
attribute
The historical way to specify optional pieces of
information about the function. The following
attributes may appear here:
isStrict
Equivalent to STRICT or RETURNS NULL ON NULL INPUT
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isCachable
isCachable is an obsolete equivalent of IMMUTABLE;
it's still accepted for backwards-compatibility
reasons.
Attribute names are not case-sensitive.
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(l)]
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
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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-
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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.
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 [drop_function(l)], GRANT [grant(l)], LOAD
[load(l)], REVOKE [revoke(l)], createlang(1), PostgreSQL
Programmer's Guide
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