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Re: Query optimizing

From: Richard Huxton <dev(at)archonet(dot)com>
To: Sebastian Ritter <ritter(dot)sebastian(at)gmail(dot)com>
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: Query optimizing
Date: 2008-11-10 12:32:42
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-sql
Sebastian Ritter wrote:
> Could it have something
> to do with the fact that it is a subquery and thus the planner can not
> deduce filtering conditions from the outer query against it? My apologises
> if that made no sense.

Could make a difference.

> In summary, what im trying to understand is the following: Will there be a
> performance difference between filtering query sets first and then joining
> them together as opposed to joining first and then filtering? Does the
> opitmiser not choose the best course of action either way yielding the same
> result?

There obviously is a performance difference between joining all of the
issues table versus joining 1% of it to followups.

In most cases the planner can push the condition into the subquery, but
not in all cases because:
 1. It's not provably correct to do so
 2. The planner isn't smart enough to figure out that it can

It's impossible to say which applies to you without knowing the full query.

  Richard Huxton
  Archonet Ltd

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