Re: Unusual slowdown using subselects

> calendar=# explain select * from schedule where schedule.owner_id in
> (select group_id from groups where user_id=101);
> NOTICE: QUERY PLAN:
> Seq Scan on schedule (cost=0.00..2039895.00 rows=1000000 width=40)
> SubPlan
> -> Materialize (cost=2.02..2.02 rows=1 width=4)
> -> Index Scan using groups_id_index on groups (cost=0.00..2.02
> rows=1 width=4)
>
> You'll see in this one, where the first example did a index scan, this one
> with a very similar query does a seq scan. The two queries should be
> nearly identical, but this one runs very slowly.
>
> Can anyone explain why this happens and/or how I can do a sub-select like
> this and get fast results?

Try using EXISTS rather than IN (see the FAQ for more information).