low performance on functions returning setof record

Hi there,

I use different functions returning setof record, and they are working well.
The problem is the performance when I use those functions in joins, for
instance:

        SELECT *
        FROM "Table1" t1
                JOIN "Function1"( a1, a2, ... aN ) AS f1( ColA int4, ColB
varchar, ... )
                        ON t1.ColX = f1.ColA

The problem is I'm not able to make indexes on the function, even inside I
have just another select statement from different permanent tables, with
some where clauses depending on the function arguments.

Do you know a way to build such a function, returning something I can join
in an outer select statement like above, using indexes or another way to run
it faster ?

TIA,
Sabin