FK index q'n

[rihad <rihad(at)mail(dot)ru>]

Given this table:

CREATE TABLE foo (
    id integer primary key,
    bar_id integer references bar (id)
);
and provided that bar.id is itself a PK, do I still need to create an 
index on bar_id if often doing queries like:
SELECT MIN(id) FROM foo WHERE bar_id IS NULL;

Table foo will contain a static number of rows (from 2,000 to 10,000 -- 
yet undecided) only doing SELECT & UPDATE.

Thanks.