From: Craig Ringer <craig@postnewspapers.com.au>
To: Decibel! <decibel@decibel.org>
Cc: Chris Kratz <chris.kratz@vistashare.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Incorrect estimates on correlated filters
Date: Thu, 14 Aug 2008 09:48:03 +0800
Message-ID: <48A38ED3.8080500@postnewspapers.com.au> (raw)
In-Reply-To: <12707C41-48C3-4072-A152-46FAB1FEE1E2@decibel.org>
References: <3642025c0808121459x75c8d9baq66194b87f5053ea3@mail.gmail.com>
<12707C41-48C3-4072-A152-46FAB1FEE1E2@decibel.org>
Decibel! wrote:
> Well... you could try and convince certain members of the community that
> we actually do need some kind of a query hint mechanism... ;)
It strikes me that there are really two types of query hint possible here.
One tells the planner (eg) "prefer a merge join here".
The other gives the planner more information that it might not otherwise
have to work with, so it can improve its decisions. "The values used in
this join condition are highly correlated".
Is there anything wrong with the second approach? It shouldn't tend to
suppress planner bug reports etc. Well, not unless people use it to lie
to the planner, and I expect results from that would be iffy at best. It
just provides information to supplement Pg's existing stats system to
handle cases where it's not able to reasonably collect the required
information.
--
Craig Ringer
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: craig@postnewspapers.com.au, decibel@decibel.org, chris.kratz@vistashare.com
Subject: Re: Incorrect estimates on correlated filters
In-Reply-To: <48A38ED3.8080500@postnewspapers.com.au>
* 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