Help with sql

Hi Guys,

This isn't a PostgreSQL specific question but just a SQL question.  If this is not an appropriate question for this
list,please let me know. 

It is also, perhaps, a really silly question.

This query (without the 'explain' keyword) , when executed takes forever and a day:

> condor_development=> explain select id from filesets where id not in ( select fileset_id from
service_pack_fileset_maps); 
>                                           QUERY PLAN
> ----------------------------------------------------------------------------------------------
>  Seq Scan on filesets  (cost=0.00..71937742.00 rows=26088 width=4)
>    Filter: (NOT (SubPlan 1))
>    SubPlan 1
>      ->  Materialize  (cost=0.00..2517.78 rows=95852 width=4)
>            ->  Seq Scan on service_pack_fileset_maps  (cost=0.00..1663.52 rows=95852 width=4)
> (5 rows)

This query returns within a second:

> condor_development=> explain select id from filesets where id not in ( select distinct fileset_id from
service_pack_fileset_maps); 
>                                           QUERY PLAN
> ----------------------------------------------------------------------------------------------
>  Seq Scan on filesets  (cost=2102.31..3153.53 rows=26088 width=4)
>    Filter: (NOT (hashed SubPlan 1))
>    SubPlan 1
>      ->  HashAggregate  (cost=1903.15..2062.48 rows=15933 width=4)
>            ->  Seq Scan on service_pack_fileset_maps  (cost=0.00..1663.52 rows=95852 width=4)
> (5 rows)

The difference is the "distinct" keyword in the inner select.

What I'm confused about is why isn't the "distinct" implicit?  I thought the construct "blah in ( select ... )" was
using"sets" and an item (I thought) can not be in a set more than once. 

Perhaps my question is the opposite really?  Why would you not always use "distinct" in the inner select when the
operatoris "in" or "not in" ? 

And if I can throw in another question on top: is there a different method other than "not in" that would work better?

Thank you guys for the help and a really awesome database.
pedz
(this is *suppose* to be sent as plain text... I hope my mailer does what it is told)