where clause + function, execution order

Hi,

I have the following function:

===============================
CREATE OR REPLACE FUNCTION xxx(text)
RETURNS SETOF vvvvv AS
$BODY$
select a.x, a.y,
CASE
WHEN strpos($1,b.x) > 0
THEN b.x
ELSE NULL
END AS mp_hm
from a LEFT JOIN b ON a.id=b.id

$BODY$
LANGUAGE sql STABLE
COST 1000
ROWS 10000;
===============================

which I call as:

select * from xxx('test0|test1') where a.x = 'value'

I am wondering when the where clause (a.x = 'value') is executed. After the select statement in the function finishes? Or is it appended at the select statement in the function?

Thank you,
Sorin