From: Alvaro Herrera <alvherre@commandprompt.com>
To: Chris Kratz <chris.kratz@vistashare.com>
Cc: Decibel! <decibel@decibel.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Incorrect estimates on correlated filters
Date: Wed, 13 Aug 2008 16:58:30 -0400
Message-ID: <20080813205830.GC4672@alvh.no-ip.org> (raw)
In-Reply-To: <3642025c0808131145x61b9aec5l2d9cb0268d28e9ff@mail.gmail.com>
References: <3642025c0808121459x75c8d9baq66194b87f5053ea3@mail.gmail.com>
<12707C41-48C3-4072-A152-46FAB1FEE1E2@decibel.org>
<3642025c0808131145x61b9aec5l2d9cb0268d28e9ff@mail.gmail.com>
Chris Kratz wrote:
> Unfortunately, if I don't think the sorting idea would help in the one case
> I'm looking at which involves filters on two tables that are joined
> together. The filters happen to be correlated such that about 95% of the
> rows from each filtered table are actually returned after the join.
> Unfortunately, the planner thinks we will get 1 row back.
Maybe you can wrap that part of the query in a SQL function and set its
estimated cost to the real values with ALTER FUNCTION ... ROWS.
--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: alvherre@commandprompt.com, chris.kratz@vistashare.com, decibel@decibel.org
Subject: Re: Incorrect estimates on correlated filters
In-Reply-To: <20080813205830.GC4672@alvh.no-ip.org>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox