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
MR: J'ai beaucoup entendu parler de fcol, mais je n'y suis jamais alle
MR: jeter un oeil....  c'est quoi l'adresse ?
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