From: Achilleus Mantzios <achill@matrix.gatewaynet.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Rod Taylor <rbt@rbt.ca>
Cc: Stephan Szabo <sszabo@megazone23.bigpanda.com>
Cc: Pgsql Performance <pgsql-performance@postgresql.org>
Cc: pgsql-general@postgresql.org
Cc: pgsql-sql@postgresql.org
Subject: Re: [SQL] [PERFORM] 7.3.1 index use / performance
Date: Wed, 8 Jan 2003 15:53:54 -0200 (GMT+2)
Message-ID: <Pine.LNX.4.44.0301081547150.14415-100000@matrix.gatewaynet.com> (raw)
In-Reply-To: <19889.1041967568@sss.pgh.pa.us>
On Tue, 7 Jan 2003, Tom Lane wrote:
> Rod Taylor <rbt@rbt.ca> writes:
> >> I am wondering about a compiler bug, or some other peculiarity on your
> >> platform. Can anyone else using FreeBSD try the above experiment and
> >> see if they get different results from mine on 7.3.* (or CVS tip)?
>
> > On FreeBSD 4.7 I received the exact same results as Tom using the
> > statements shown by Tom.
>
> On looking at the code, I do see part of a possible mechanism for this
> behavior: cost_index calculates the estimated cost for qual-clause
> evaluation like this:
>
This bizarre index decreased cost (when adding conditions) behaviour maybe
was due to some vacuums.
(i cant remember how many reloads and vacuums i did to the database
in the period petween the two emails).
However my linux machine with the same pgsql 7.3.1, with a full clean
installation also gives the same symptoms:
Choosing the slow index, and after some (random)
vacuums choosing the right index, and then after some vacuums chooses the
bad
index again.
>
> regards, tom lane
>
==================================================================
Achilleus Mantzios
S/W Engineer
IT dept
Dynacom Tankers Mngmt
Nikis 4, Glyfada
Athens 16610
Greece
tel: +30-10-8981112
fax: +30-10-8981877
email: achill@matrix.gatewaynet.com
mantzios@softlab.ece.ntua.gr
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To: pgsql-sql@postgresql.org
Cc: achill@matrix.gatewaynet.com, tgl@sss.pgh.pa.us, rbt@rbt.ca, sszabo@megazone23.bigpanda.com, pgsql-performance@postgresql.org, pgsql-general@postgresql.org
Subject: Re: [SQL] [PERFORM] 7.3.1 index use / performance
In-Reply-To: <Pine.LNX.4.44.0301081547150.14415-100000@matrix.gatewaynet.com>
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