From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Andres Freund <andres@anarazel.de>,
        Claudio Freire <klaussfreire@gmail.com>,
        =?UTF-8?B?0K7RgNC40Lkg0KHQvtC60L7Qu9C+0LI=?= <funny.falcon@gmail.com>,
        Sokolov Yura <funny.falcon@postgrespro.ru>,
        Alvaro Herrera <alvherre@2ndquadrant.com>,
        Heikki Linnakangas <hlinnaka@iki.fi>,
        PostgreSQL-Dev <pgsql-hackers@postgresql.org>
Subject: Re: Small improvement to compactify_tuples
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Wed, 08 Nov 2017 06:13:53 -0500"
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Date: Wed, 08 Nov 2017 10:33:19 -0500
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Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Nov 7, 2017 at 4:39 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> What I'm getting from the standard pgbench measurements, on both machines,
>> is that this patch might be a couple percent slower than HEAD, but that is
>> barely above the noise floor so I'm not too sure about it.

> Hmm.  It seems like slowing down single client performance by a couple
> of percent is something that we really don't want to do.

I do not think there is any change here that can be proven to always be a
win.  Certainly the original patch, which proposes to replace an O(n log n)
sort algorithm with an O(n^2) one, should not be thought to be that.
The question to focus on is what's the average case, and I'm not sure how
to decide what the average case is.  But more than two test scenarios
would be a good start.

			regards, tom lane


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