Received: from localhost (unknown [200.46.204.183]) by developer.postgresql.org (Postfix) with ESMTP id 18F542E00AF for ; Thu, 12 Jun 2008 14:03:28 -0300 (ADT) Received: from developer.postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.204.183]) (amavisd-maia, port 10024) with ESMTP id 90919-02-8 for ; Thu, 12 Jun 2008 14:03:22 -0300 (ADT) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130]) by developer.postgresql.org (Postfix) with ESMTP id 6ED4E2E0152 for ; Thu, 12 Jun 2008 13:52:27 -0300 (ADT) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss.pgh.pa.us (8.14.2/8.14.2) with ESMTP id m5CGqQZ4010802; Thu, 12 Jun 2008 12:52:26 -0400 (EDT) To: "Greg Sabino Mullane" cc: pgsql-hackers@postgresql.org Subject: Re: Overhauling GUCS In-reply-to: References: Comments: In-reply-to "Greg Sabino Mullane" message dated "Thu, 12 Jun 2008 16:20:56 -0000" Date: Thu, 12 Jun 2008 12:52:26 -0400 Message-ID: <10801.1213289546@sss.pgh.pa.us> From: Tom Lane X-Virus-Scanned: Maia Mailguard 1.0.1 X-Archive-Number: 200806/555 X-Sequence-Number: 119668 "Greg Sabino Mullane" writes: > The orders of magnitude speed up of certain queries when the d_s_t goes > above 98 is what spawned my original thread proposing a change to 100: > http://markmail.org/message/tun3a3juxlsyjbsw That was a pretty special case (LIKE/regex estimation), and we've since eliminated the threshold change in the LIKE/regex estimates anyway, so there's no longer any reason to pick 100 as opposed to any other number. So we're still back at "what's a good value and why?". > Frankly, I'd be shocked if there is any significant difference and all > compared to the actual query run time. I'm still concerned about the fact that eqjoinsel() is O(N^2). Show me some measurements demonstrating that a deep nest of equijoins doesn't get noticeably more expensive to plan --- preferably on a datatype with an expensive equality operator, eg numeric --- and I'm on board. regards, tom lane