From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: Franck Routier <franck.routier@axege.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: What happens between end of explain analyze and end of query execution ?
Date: Mon, 08 Apr 2013 12:46:26 -0400
Message-ID: <16275.1365439586@sss.pgh.pa.us> (raw)
In-Reply-To: <CAMkU=1zSCsB1oPRsK65Y4JV1+AWwDtLyR26NTHCRpfqSk_ORZw@mail.gmail.com>
References: <515D92BF.5000309@axege.com>
<CAPmjWd100eemBz1RTw=r24zgh8+YO8iq86fThqsALznrPuyWqg@mail.gmail.com>
<515D9FC5.7030105@axege.com>
<18910.1365092713@sss.pgh.pa.us>
<515DCB34.6040801@axege.com>
<6354.1365102532@sss.pgh.pa.us>
<515ED7BC.40907@axege.com>
<4113.1365171448@sss.pgh.pa.us>
<515FE547.3010301@axege.com>
<5541.1365265641@sss.pgh.pa.us>
<CAMkU=1zSCsB1oPRsK65Y4JV1+AWwDtLyR26NTHCRpfqSk_ORZw@mail.gmail.com>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-performance>
Jeff Janes <jeff.janes@gmail.com> writes:
> On Sat, Apr 6, 2013 at 9:27 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Yeah, eqjoinsel() is O(N^2) in the lengths of the MCV lists, in the
>> worst case where there's little overlap in the list memberships.
> I guess this pre-emptively answers a question I was intending to ask on
> performance: Whether anyone increased default_statistics_target and came
> to regret it. I had seen several problems fixed by increasing
> default_statistics_target, but this is the first one I recall caused by
> increasing it.
I recall having heard some similar complaints before, but not often.
> Do you think fixing the O(N^2) behavior would be a good to-do item?
If you can think of a way to do it that doesn't create new assumptions
that eqjoinsel ought not make (say, that the datatype is sortable).
I guess one possibility is to have a different join selectivity function
for those types that *are* sortable, which would fix the issue for most
commonly used types.
regards, tom lane
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
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: tgl@sss.pgh.pa.us, jeff.janes@gmail.com, franck.routier@axege.com
Subject: Re: What happens between end of explain analyze and end of query execution ?
In-Reply-To: <16275.1365439586@sss.pgh.pa.us>
* 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