Timestamps and performances problems

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
> Notre devoir, pour leur bien et pour le bien de Linux, est de te
> flinguer avant que tu ne les sacrifies (fût-ce avec les meilleures
> intentions du monde).
-+- TP In: Guide du linuxien pervers : "De la pédagogie par l'Exemple"