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