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From: Jesper Krogh <jesper@krogh.cc>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Rauan Maemirov <rauan@maemirov.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: pgsql-performance@postgresql.org
Subject: Re: Problems with FTS
Date: Thu, 01 Dec 2011 07:11:40 +0100
Message-ID: <4ED71A9C.5030603@krogh.cc> (raw)
In-Reply-To: <CA+Tgmoaqy-O9OWYnO-Xc0ei_mytB6C2rMAarjwuM+581TGRaLA@mail.gmail.com>
References: <4D0CA1890200002500038774@gw.wicourts.gov>
	<AANLkTim4vLhMB5Ty6hTN4XdNoJP0BWYxguqEXt0+r920@mail.gmail.com>
	<AANLkTinAKawNNfs8VKpqrCJzyfn=EQQh=6XqcF4iy4ur@mail.gmail.com>
	<CAFw_bqOdQqE0fUVntCX-aV+oZnzEgpmGqce=ODktJkNktFEEBQ@mail.gmail.com>
	<CA+Tgmoaqy-O9OWYnO-Xc0ei_mytB6C2rMAarjwuM+581TGRaLA@mail.gmail.com>

On 2011-11-30 21:58, Robert Haas wrote:
> The row-count estimates look reasonably accurate, so there's some
> other problem here.  What do you have random_page_cost, seq_page_cost,
> and effective_cache_size set to?  You might try "SET
> random_page_cost=2" or even "SET random_page_cost=0.5; SET
> seq_page_cost=0.3" and see if those settings help
I may be seing ghosts here, since I've encountered
the same problem.  But the Query-planner does not
take toast into account, so a Sequential Scan + filter
only cost what it takes to scan the main table, but fts-fields
are typically large enough to be toasted so the cost should
be main+toast (amount of pages) + filtering cost.

I posted about it yesterday:

http://archives.postgresql.org/pgsql-hackers/2011-11/msg01754.php

If above problem is on <9.1 a patch to proper account of gin-estimates
have been added to 9.1 which also may benefit the planning:
http://www.postgresql.org/docs/9.1/static/release-9-1.html

     Improve GIN index scan cost estimation (Teodor Sigaev)

Jesper
-- 
Jesper



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