proposal: schema private functions

Hi

I would to introduce new flag for routines - PRIVATE. Routines with this
flag can be called only from other routines assigned with same schema.
Because these routines are not available for top queries, we can hide these
functions from some outputs like \df ..

Example:

CREATE SCHEMA s1;
CREATE SCHEMA s2;

CREATE OR REPLACE FUNCTION s1.nested()
RETURNS int AS $$
BEGIN
RETURN random()*1000;
END;
$$ PRIVATE;

SELECT s1.nested(); -- fails - it is top query

CREATE OR REPLACE FUNCTION s1.fx()
RETURNS int AS $$
BEGIN
RETURN s1.nested();
END;
$$ LANGUAGE plpgsql;

SELECT s1.fx(); -- will work

CREATE OR REPLACE FUNCTION s2.fx()
RETURNS int AS $$
BEGIN
RETURN s1.nested();
END;
$$ LANGUAGE plpgsql;

SELECT s2.fx(); -- fails - it call private function from other schema.

This proposal is simple, and strong enough to separate functions that can
be directly callable and auxiliary functions, that can be called from other
functions.

I wrote PoC implementation, and it is not hard, and it should not to impact
performance. I introduced query_owner_nspid into query environment. When
any functions has flag private, then query_owner_nspid should be same like
function namespace id.

Comments, notes?

Regards

Pavel