Does IMMUTABLE have any effect on functions?

Hi,

i wonder if the attribute IMMUTABLE has any effect on functions.
Maybe its not implemented yet?

I tried the following:

CREATE TABLE foo (bar int4);

CREATE FUNCTION foo(int4)
RETURNS int4
AS '
INSERT INTO foo (bar) VALUES ($1);
SELECT $1;
' LANGUAGE 'sql' IMMUTABLE;

...now without any transaction...

select * from foo(1);

foo
-----
1
(1 row)

SELECT * FROM foo;

bar
-----
1
(1 row)

select * from foo(1);

foo
-----
1
(1 row)

SELECT * FROM foo;

bar
-----
1
1
(1 row)

In my expectations the 2nd function call should not have added a new row
to table "foo", beacause it was called with the same parameter and is
immutable.

The same happens when i try to call the function twice within a single
transaction.

Maybe the "IMMUTABLE" attribute is just some sort of comment?

greets, Tom Schön