Re: Timestamps and performances problems

On 10 Apr 2002 at 10:44, JX wrote:

> Le Wed, 10 Apr 2002 09:27:09 -0400
> "Dan Langille" <dan@langille.org> me disait que :
>
> > On 10 Apr 2002 at 9:13, JX wrote:
> >
> > > Le Wed, 10 Apr 2002 09:06:55 -0400
> > > "Dan Langille" <dan@langille.org> me disait que :
> > >
> > > > On 10 Apr 2002 at 11:51, Gaetano Mendola wrote:
> > > >
> > > > > "Jean-Christophe ARNU (JX)" <jc.arnu@free.fr> wrote:
> > > > > > 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)
> > > >
> > > > Try where timestamp<now() and timestamp>(now() - '1
> > > > hour'::interval)::timestemp.
> > >
> > >     What's the difference with the syntax above? It takes he same time
> > >     than the query above. Bounded timestamps with "real" ISO timestamps
> > >     strings are always up to about 200 times faster (with extensive test
> > >     proof).
> >
> > It casts the value to a timestamp.  I would prefer to discuss this on-
> > list.
>     Okaye, but what's the incidence on preformance issues?
>     Casting should only insure that given string is to be taken as a timestamp
> isn't it? Does it make an "instanciation" of the timestamp to be that would be
> applied for comparision clauses?

If there is an index on that field, casting to a timestamp may help the
optimization.  Therefore I suggested that it be tried.
--
Dan Langille
The FreeBSD Diary - http://freebsddiary.org/ - practical examples