Timestamps and performance problems on queries.

Hello all.
I've a performance problem on specific requests :

When I use timestamps + interval in where clauses, query performance is slowed
down by a factor of 20 or 30!!!! For exemple :
    select timestamp,value
    from measure
    where timestamp<now() and timestamp>(now() - '1 hour'::interval)

    is 20 to 30 times longer than

    select timestamp,value
    from measure
    where timestamp<'2002-04-10 10:00' and timestamp>='2002-04-10 9:00';

So where is the bottleneck?
A paradigm seems that now() and (now() - '1hour'::interval) is evaluated for
each row comparison... Am I right? Thus is there a way to make SQL interpreter
evaluate this by rewriting them before launching any comparisons?

Or do I have to rewrite all my application queries and calculate each time
now() and interval predicates?

    Thanks by advance

--
Jean-Christophe ARNU
s/w developer
Paratronic France
> (Pas Adabase que j'ai en version démo sur la distribution Suze).
Et après ça, allez expliquer que les linuxiens sont pas alcolos... ;-)
à quand une distribution Kro v1.6.64 ou Pastis v5.1?
-+-  Fantec In Guide du linuxien pervers - "C'est l'heure de l'apéro ?" -+-