Expression indexes and dependecies

Hello,

While doing some tests, I observed that expression indexes can malfunction
if the underlying expression changes. For example, say I define a function
foo() as:

CREATE OR REPLACE FUNCTION foo(a integer) RETURNS integer AS $$
BEGIN
RETURN $1 + 1;
END;
$$ LANGUAGE plpgsql IMMUTABLE;

I then create a table, an expression index on the table and insert a few
rows:

CREATE TABLE test (a int, b char(20));
CREATE UNIQUE INDEX testindx ON test(foo(a));
INSERT INTO test VALUES (generate_series(1,10000), 'bar');

A query such as following would return result using the expression index:

SET enable_seqscan TO off;
SELECT * FROM test WHERE foo(a) = 100;

It will return row with a = 99 since foo() is defined to return (a + 1)

If I now REPLACE the function definition with something else, say to return
(a + 2):

CREATE OR REPLACE FUNCTION foo(a integer) RETURNS integer AS $$
BEGIN
RETURN $1 + 2;
END;
$$ LANGUAGE plpgsql IMMUTABLE;

I get no error/warnings, but the index and the new function definition are
now out of sync. So above query will still return the same result, though
the row with (a = 99) no longer satisfies the current definition of
function foo().

Perhaps this is a known behaviour/limitation, but I could not find that in
the documentation. But I wonder if it makes sense to check for dependencies
during function alteration and complain. Or there are other reasons why we
can't do that and its a much larger problem than what I'm imagining ?

Thanks,
Pavan

--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee