how to optimize: max(timetstamp) where a.foo=b.foo ?

[walter harms]

hi list,
i have tables that look like this( <10.000 entries) :

id,
timestamp,
value


to get the latest value for each id i have queries like:

select * from tab A where timestamp = (select max(timestamp) from tab B where B.id=A.id)
group by id ;

on a fast system it takes round 4 sec to complet, but since this is done requlary it slows
down to much.
the same query takes on a informix system an a slower computer next to nothing.

any ideas ?

re,
 walter