Subqueries and indexes

In this QUERY:

SELECT keyname
FROM markmain
WHERE mark_id NOT IN(SELECT mark_id
FROM markaty)

I have an index on markaty.mark_id, and have vacuum analyzed. EXPLAIN
shows:

Seq Scan on markmain (cost=2051.43 size=45225 width=12)
SubPlan
-> Seq Scan on markaty (cost=2017.41 size=52558 width=4)

Vadim, why isn't this using the index? Each table has 50k rows. Is it
NOT IN that is causing the problem? IN produces the same plan, though.
If I do a traditional join:

SELECT keyname
FROM markmain , markaty
WHERE markmain.mark_id = markaty.mark_id

I then get a hash join plan:

Hash Join (cost=10768.51 size=90519 width=20)
-> Seq Scan on markmain (cost=2051.43 size=45225 width=16)
-> Hash (cost=0.00 size=0 width=0)
-> Seq Scan on markaty (cost=2017.41 size=52558 width=4)

Seems the optimizer could either hash the subquery, or us an index.
Certainly would be faster than a sequental scan, no?

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