How does the planner treat a table function.

Hi,
we have got some tables (uw?) and functions. One function is defined like

get_abc():
SELECT a,b,c from table_x;

What happens if I query something like
SELECT a,b from get_abc() where a=5;
while table_x is rather big?

Will PSQL at first query all records of table_x and then apply a where a=5
OR
will PSQL integrate it to a shorter query?

(In case there will be of course an intex over a to prevent sequential
search)