Subqueries and indexes

In this QUERY:
SELECT keynameFROM markmainWHERE 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=45225width=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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