Timestamps and performances problems

From: Jean-Christophe ARNU (JX) <arnu(at)paratronic(dot)fr>
To: pgsql-admin(at)postgresql(dot)org
Subject: Timestamps and performances problems
Date: 2002-04-10 08:22:15
Message-ID: 20020410042215.38c9f863.arnu@paratronic.fr
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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"

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