function optimization ???

Hi,

I've the following function:

CREATE FUNCTION book_info(pricing)
RETURNS catalog_general AS '
select *
from catalog_general
where star_isbn = $1.vista_isbn
' LANGUAGE 'sql';

calling it as:

SELECT p.*, p.book_info.title FROM pricing p WHERE vista_ans='POD';

background and observation:

the pricing table is fairly large, but only a small number meet
"WHERE vista_ans='POD'". I can select all where vista_ans='POD'
very quickly (.2 sec), but adding in the get_book(pricing) call
slows this down to about 20sec. I can, with an external sql query,
select all of the desired records in about 1 sec, so it appears
to me that the function is being called regardless of whether
or not the WHERE clause is being satisfied.

question:

is there any way the function call could be _not_ called if:
1) the WHERE clause does not reference any of its return values, and
2) the WHERE clause has already been satisified.

???

If this behavior is reasonable, could someone point me _toward_ the
code where I'd need to make this optimization. I think this would be
nice to have for 7.2 :)

brent