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From: Floris Van Nee <florisvannee@Optiver.com>
To: Dmitry Dolgov <9erthalion6@gmail.com>
To: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Cc: Jeff Janes <jeff.janes@gmail.com>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Jesper Pedersen <jesper.pedersen@redhat.com>
Cc: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Cc: Peter Geoghegan <pg@bowt.ie>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Bhushan Uparkar <bhushan.uparkar@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>
Cc: James Coleman <jtc331@gmail.com>
Subject: Re: Index Skip Scan
Date: Sat, 1 Jun 2019 04:10:23 +0000
Message-ID: <1559362223126.18661@Optiver.com> (raw)
In-Reply-To: <1559361698401.22864@Optiver.com>
References: <2685f020-f302-9bf6-1b5c-ebc2bcebc635@2ndquadrant.com>
	<CA+q6zcUSMQtDVr-isZkm8FP4Mh6APkNyzkPYQPr1ZugeDV2XPA@mail.gmail.com>
	<CA+q6zcUSuFBhGVFZN_AVSxRbt5wr_4_YEYwv8PcQB=m6J6Zpvg@mail.gmail.com>
	<20190315.125452.252656146.horiguchi.kyotaro@lab.ntt.co.jp>
	<CA+q6zcXcSftAxfwuK5c186E8ckZeWmOO07GFvpoOx-wpsDGGzA@mail.gmail.com>
	<CA+q6zcVn10QzbSh686PaZpxgxMhxABRj7aJJpsJ=tKkmcSrBew@mail.gmail.com>
	<CA+q6zcUVAnfSOaSk3s9Sbj6yC7LrDYChSyJd0t7u6Q=97k5a=A@mail.gmail.com>
	<CA+q6zcVQG0ufA_5m_7eeHLN2VRy+z8Kf9FoZ2NnF=yd0_43ndg@mail.gmail.com>
	<CA+q6zcXxwU1FZ=Tj0kWeTkrJ1N9=fhrUH1t5N_yHEcKsG=+qUw@mail.gmail.com>
	<1559361698401.22864@Optiver.com>

Actually I'd like to add something to this. I think I've found a bug in the current implementation. Would someone be able to check?

Given a table definition of (market text, feedcode text, updated_at timestamptz, value float8) and an index on (market, feedcode, updated_at desc) (note that this table slightly deviates from what I described in my previous mail) and filling it with data.


The following query uses an index skip scan and returns just 1 row (incorrect!)

select distinct on (market, feedcode) market, feedcode
from streams.base_price
where market='TEST'

The following query still uses the regular index scan and returns many more rows (correct)
select distinct on (market, feedcode) *
from streams.base_price
where market='TEST'


It seems that partially filtering on one of the distinct columns triggers incorrect behavior where too many rows in the index are skipped.


-Floris

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  Cc: florisvannee@Optiver.com, 9erthalion6@gmail.com, horiguchi.kyotaro@lab.ntt.co.jp, jeff.janes@gmail.com, thomas.munro@gmail.com, jesper.pedersen@redhat.com, a.kuzmenkov@postgrespro.ru, pg@bowt.ie, tomas.vondra@2ndquadrant.com, thomas.munro@enterprisedb.com, bhushan.uparkar@gmail.com, a.korotkov@postgrespro.ru, jtc331@gmail.com
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