From: Tom Lane <tgl@sss.pgh.pa.us>
To: Martijn van Oosterhout <kleptog@svana.org>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: Kelly Burkhart <kelly.burkhart@gmail.com>
Cc: Evgeny Gridasov <eugrid@fpm.kubsu.ru>
Cc: pgsql-performance@postgresql.org
Cc: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] EXPLAIN ANALYZE on 8.2
Date: Fri, 15 Dec 2006 09:56:57 -0500
Message-ID: <5822.1166194617@sss.pgh.pa.us> (raw)
In-Reply-To: <20061215123052.GJ958@svana.org>
References: <20061214163230.d46f0138.eugrid@fpm.kubsu.ru>
<21146.1166112702@sss.pgh.pa.us>
<20061214192920.c75385d6.eugrid@fpm.kubsu.ru>
<21516.1166114475@sss.pgh.pa.us>
<fa1e4ce70612141543o8866d7fg417eca88ce9b1344@mail.gmail.com>
<29450.1166141131@sss.pgh.pa.us>
<1166178488.3882.56.camel@silverbirch.site>
<20061215105008.GD958@svana.org>
<1166185247.3882.69.camel@silverbirch.site>
<20061215123052.GJ958@svana.org>
Martijn van Oosterhout <kleptog@svana.org> writes:
> On Fri, Dec 15, 2006 at 12:20:46PM +0000, Simon Riggs wrote:
>> Maybe sampling every 10 rows will bring things down to an acceptable
>> level (after the first N). You tried less than 10 didn't you?
> Yeah, it reduced the number of calls as the count got larger. It broke
> somewhere, though I don't quite remember why.
The fundamental problem with it was the assumption that different
executions of a plan node will have the same timing. That's not true,
in fact not even approximately true. IIRC the patch did realize
that first-time-through is not a predictor for the rest, but some of
our plan nodes have enormous variance even after the first time.
I think the worst case is batched hash joins.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, kleptog@svana.org, simon@2ndquadrant.com, kelly.burkhart@gmail.com, eugrid@fpm.kubsu.ru, pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] EXPLAIN ANALYZE on 8.2
In-Reply-To: <5822.1166194617@sss.pgh.pa.us>
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