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Re: Posible planner improvement?

From: Albert Cervera Areny <albert(at)sedifa(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Posible planner improvement?
Date: 2008-05-21 11:37:49
Message-ID: 200805211337.49472.albert@sedifa.com (view raw or flat)
Thread:
Lists: pgsql-performance
A Dimecres 21 Maig 2008, Albert Cervera Areny va escriure:
> A Dimecres 21 Maig 2008, Mark Mielke va escriure:
> > A Dimecres 21 Maig 2008, Richard Huxton va escriure:
> > >> Albert Cervera Areny wrote:
> > >>> I've got a query similar to this:
> > >>>
> > >>> select * from t1, t2 where t1.id > 158507 and t1.id = t2.id;
> > >>>
> > >>> That took > 84 minutes (the query was a bit longer but this is the
> > >>> part that made the difference) after a little change the query took
> > >>> ~1 second:
> > >>>
> > >>> select * from t1, t2 where t1.id > 158507 and t2.id > 158507 and
> > >>> t1.id = t2.id;
> > >>
> > >> Try posting EXPLAIN ANALYSE SELECT ... for both of those queries and
> > >> we'll see why it's better at the second one.
> >
> > Even if the estimates were off (they look a bit off for the first
> > table), the above two queries are logically identical, and I would
> > expect the planner to make the same decision for both.
> >
> > I am curious - what is the result of:
> >
> >     select * from t1, t2 where t2.id > 158507 and t1.id = t2.id;
> >
> > Is it the same speed as the first or second, or is a third speed
> > entirely?
>
> Attached the same file with the third result at the end. The result is
> worst than the other two cases. Note that I've analyzed both tables but
> results are the same. One order of magnitude between the two first queries.

Sorry, it's not worse than the other two cases as shown in the file. However, 
after repetition it seems the other two seem to decrease more than the third 
one whose times vary a bit more and some times take up to 5 seconds.

Other queries are running in the same machine, so take times with a grain of 
salt. What's clear is that always  there's a big difference between first and 
second queries.

>
> > If t1.id = t2.id, I would expect the planner to substitute them freely
> > in terms of identities?
> >
> > Cheers,
> > mark



-- 
Albert Cervera Areny
Dept. Informàtica Sedifa, S.L.

Av. Can Bordoll, 149
08202 - Sabadell (Barcelona)
Tel. 93 715 51 11
Fax. 93 715 51 12

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