Speeding up SELECT MAX(),... GROUP BY ... ?

Hi,

I am having select statements which are running too slow.
It's PostgreSQL 7.2.1 and a VACUUM ANALYZE was done.
There are various where clauses restricting the output
to timepoints and/or ids related to sources,locations,...
One statement looks like:

EXPLAIN
select max(timepoint),lid,mid,sid from onfvalue
where timepoint <= '2002-05-01 08:00:00' and sid in (1,3,5,7) group by sid,lid,mid;

NOTICE: QUERY PLAN:

Aggregate (cost=21044.07..22383.53 rows=13395 width=20)
-> Group (cost=21044.07..22048.66 rows=133946 width=20)
-> Sort (cost=21044.07..21044.07 rows=133946 width=20)
-> Seq Scan on onfvalue (cost=0.00..5530.24 rows=133946 width=20)

Any ideas/alternatives to speed this up?
The table 'onfvalue' is supposed to keep some million rows.

Regards, Christoph