Received: from malur.postgresql.org ([217.196.149.56]) by arkaria.postgresql.org with esmtp (Exim 4.72) (envelope-from ) id 1UOoSr-0000NR-9f for pgsql-performance@arkaria.postgresql.org; Sun, 07 Apr 2013 12:12:53 +0000 Received: from localhost ([127.0.0.1] helo=postgresql.org) by malur.postgresql.org with smtp (Exim 4.72) (envelope-from ) id 1UOoSq-00022G-Kg for pgsql-performance@arkaria.postgresql.org; Sun, 07 Apr 2013 12:12:52 +0000 Received: from magus.postgresql.org ([2a02:c0:301:0:ffff::29]) by malur.postgresql.org with esmtp (Exim 4.72) (envelope-from ) id 1UOoSp-00022A-HQ for pgsql-performance@postgresql.org; Sun, 07 Apr 2013 12:12:51 +0000 Received: from smtp02.smtpout.orange.fr ([80.12.242.124] helo=smtp.smtpout.orange.fr) by magus.postgresql.org with esmtp (Exim 4.72) (envelope-from ) id 1UOoSe-0006sp-2l for pgsql-performance@postgresql.org; Sun, 07 Apr 2013 12:12:51 +0000 Received: from axege.com ([82.127.232.162]) by mwinf5d56 with ME id M0Cd1l0023WtTCJ030Cd9g; Sun, 07 Apr 2013 14:12:38 +0200 Received: from localhost (localhost [127.0.0.1]) by axege.com (Postfix) with ESMTP id DE41043DE03 for ; Sun, 7 Apr 2013 14:12:36 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at myhostname="axege.com." Received: from axege.com ([127.0.0.1]) by localhost (axege.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id NM7KtusoKoWf for ; Sun, 7 Apr 2013 14:12:35 +0200 (CEST) Received: from [10.0.1.8] (lns-bzn-48f-81-56-219-232.adsl.proxad.net [81.56.219.232]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by axege.com (Postfix) with ESMTPSA id 8D0FC43DE02 for ; Sun, 7 Apr 2013 14:12:34 +0200 (CEST) Message-ID: <516162B1.8050909@axege.com> Date: Sun, 07 Apr 2013 14:12:33 +0200 From: Franck Routier User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130404 Thunderbird/17.0.5 MIME-Version: 1.0 To: pgsql-performance@postgresql.org Subject: Re: What happens between end of explain analyze and end of query execution ? References: <515D92BF.5000309@axege.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> In-Reply-To: <5541.1365265641@sss.pgh.pa.us> Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary="------------ms010001060502090504030600" X-Pg-Spam-Score: -1.1 (-) List-Archive: List-Help: List-ID: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: X-Mailing-List: pgsql-performance Precedence: bulk Sender: pgsql-performance-owner@postgresql.org Ceci est un message signé cryptographiquement au format MIME. --------------ms010001060502090504030600 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: quoted-printable Le 06/04/2013 18:27, Tom Lane a =E9crit : >> Ok, the problem definitely comes from the default_statistics_target >> which is obviously too high on the database. > 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. > The actual cost would depend a lot on the specific column datatypes. > > Not sure about your report that re-analyzing with the same stats target= > made a significant difference. It might have been a matter of chance > variation in the sampled MCV list leading to more or fewer matches. > > regards, tom lane > > Thank you all for your help, I appreciate it really much. Just a last note: maybe the documentation could draw the attention on=20 this side effect of high statistics target. Right now it says: "Larger values increase the time needed to do ANALYZE, but might=20 improve the quality of the planner's estimates"=20 (http://www.postgresql.org/docs/9.2/static/runtime-config-query.html#GUC-= DEFAULT-STATISTICS-TARGET) and: "Raising the limit might allow more accurate planner estimates to=20 be made, particularly for columns with irregular data distributions, at=20 the price of consuming more space in pg_statistic and slightly more time = to compute the estimates."=20 (http://www.postgresql.org/docs/9.2/static/planner-stats.html). It could be noted that a too high target can also have a noticeable cost = on query planning. 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