To: Gerhard Wiesinger <lists@wiesinger.com>
cc: Pavel Stehule <pavel.stehule@gmail.com>,
	Andreas Kretschmer <akretschmer@spamfence.net>,
	pgsql-performance@postgresql.org
Subject: Re: Major performance problem after upgrade from 8.3 to 8.4 
In-reply-to: <alpine.LFD.2.01.1008301808330.31277@bbs.intern> 
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	<alpine.LFD.2.01.1008300956320.19598@bbs.intern>
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Comments: In-reply-to Gerhard Wiesinger <lists@wiesinger.com>
	message dated "Mon, 30 Aug 2010 18:11:36 +0200"
Date: Mon, 30 Aug 2010 12:22:53 -0400
Message-ID: <11111.1283185373@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Gerhard Wiesinger <lists@wiesinger.com> writes:
> I know the drawbacks of an EAV design but I don't want to discuss that. I 
> want to discuss the major performance decrease of PostgreSQL 8.3 
> (performance was ok) to PostgreSQL 8.4 (performance is NOT ok).

> Any further ideas how I can track this down?
> Can someone explain the difference in query plan from an optimizer point 
> of view?

Since you haven't shown us the 8.3 plan, it's kind of hard to speculate ;-)

One thing that jumped out at me was that 8.4 appears to be expecting
multiple matches in each of the left-joined tables, which is why the
total rowcount estimate balloons so fast.  I rather imagine that you are
expecting at most one match in reality, else the query isn't going to
behave nicely.  Is this correct?  Are you *sure* you analyzed all these
tables?  And if that is how the data looks, where is the actual
performance problem?  A bad rowcount estimate isn't in itself going
to kill you.

FWIW, in a similar albeit toy example, I don't see any difference
between the 8.3 and 8.4 plans or cost estimates.

			regards, tom lane