From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200310240409.h9O49HE04743@candle.pha.pa.us>
Subject: Re: 7.4 compatibility question
In-Reply-To: <87brs71dpw.fsf@stark.dyndns.tv>
To: Greg Stark <gsstark@mit.edu>
Date: Fri, 24 Oct 2003 00:09:17 -0400 (EDT)
Cc: pgsql-hackers@postgresql.org
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Greg Stark wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> 
> > Michael Brusser wrote:
> >
> > > Optimizer improvements
> > >  => this tells me nothing. I suppose this could be a minor internal code
> > > tweak, which does not affect me. On the other hand this could be a major
> > > breakthrough, so now I can run some stupid query which would take
> > > a week to complete in the previous release. How do I know? 
> > 
> > Yes, this is always very hard to explain. The optimizer itself is
> > complex, and uses complex terms like merge join and key pruning.  It is
> > hard to explain what queries will be affected, though the basic issue is
> > that the optimizer will choose a better plan more frequently.
> 
> One thing that might be worth mentioning is that "WHERE foo IN (subquery)"
> type queries are much improved. That's a one of the more common complaints
> about 7.3 and previous and it's one that fairly easy to recognize.

That is right at the top of the release notes:

   Performance
          IN/NOT IN subqueries are now much more efficient

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