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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Surafel Temesgen <surafel3000@gmail.com>
Cc: David Steele <david@pgmasters.net>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: andrew@tao11.riddles.org.uk, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: FETCH FIRST clause WITH TIES option
Date: Wed, 3 Apr 2019 21:40:59 +0200
Message-ID: <20190403194059.yqtuuwtsipq2vknh@development> (raw)
In-Reply-To: <32413.1554318485@sss.pgh.pa.us>
References: <CALAY4q-+CKiFkgCL5VSK-9P8i5Z9uoQQh6ZEZLkjzsUT2mySsw@mail.gmail.com>
	<20190204052857.GP29064@paquier.xyz>
	<CALAY4q8vDyK4_jhFkXyWnJRcuWGgkwOzJTw-j2Dw0ddyTiZM4g@mail.gmail.com>
	<cd96a537-a302-48ef-b41b-2e247e156745@pgmasters.net>
	<CALAY4q8X-17+AFaAKZ+YX781b+pqjdU4S=z=ZUGi3QjyHpZ6hQ@mail.gmail.com>
	<20190329005648.GA1136@development>
	<20190331001446.GA10804@development>
	<CALAY4q86X1rCKyNE040tx1nXgH5BGeH4nxdtVbEdUuvVxEBdRw@mail.gmail.com>
	<20190403190257.44fxttkpo4yopp3l@development>
	<32413.1554318485@sss.pgh.pa.us>

On Wed, Apr 03, 2019 at 03:08:05PM -0400, Tom Lane wrote:
>Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> I've tried to fix the merge conflict (essentially by moving some of the
>> code to adjust_limit_rows_costs(), but I'm wondering if the code added to
>> create_limit_path is actually correct
>> ...
>> Firstly, this seriously needs some comment explaining why we do this.
>
>I've not looked at this patch, but TBH I wonder why it is touching
>planner rowcount estimation at all.  I find it doubtful either that
>a correction for WITH TIES would be significant in most use-cases,
>or that we could estimate it accurately if it was significant.
>It certainly doesn't seem like something that needs to be messed
>with in v1 of the feature.
>

FWIW it was me who suggested to tweak the cardinality estimation this way,
but if we want to leave it out from v1, I'm OK with that.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services






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