WITH <table> SELECT * FROM function(<table>) WHERE etc

Dear list,
Say, I wish to have a function returning a record, taking a row as
parameter. However, from a performance perspective, I wish to be able to
perform this query only on a subset by using a WHERE-clause.

I can only think of the following approach:

WITH ss AS ( SELECT * FROM my_table) SELECT * FROM my_function(ss) WHERE
ss.field1 > 100

Question is: does the optimizer apply this WHERE-clause to the WITH-SELECT
(resulting in "ss")?

Rob