Index Usage using IN

Hi,

I have 2 tables both have an index on ID (both ID columns are an oid).

I want to find only only rows in one and not the other.

Select ID from TableA where ID not IN ( Select ID from Table B)

This always generates sequential scans.

Table A has about 250,000 rows.   Table B has about 250,000 Rows.

We should get a Scan on Table B and a Index Lookup on Table A.

Is there any way to force this?  enable_seqscan off doesn't help at all.

The Plan is

Seq Scan on tablea(cost=100000000.00..23883423070450.96 rows=119414 width=4)
  Filter: (NOT (subplan))"
  SubPlan    ->
     Seq Scan on tableb (cost=100000000.00..100004611.17 rows=242617
width=4)


Thanks
Ralph