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From: Floris Van Nee <florisvannee@Optiver.com>
To: Dmitry Dolgov <9erthalion6@gmail.com>
To: David Rowley <david.rowley@2ndquadrant.com>
To: James Coleman <jtc331@gmail.com>
To: jesper.pedersen@redhat.com <jesper.pedersen@redhat.com>
Cc: Rafia Sabih <rafia.pghackers@gmail.com>
Cc: Jeff Janes <jeff.janes@gmail.com>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Peter Geoghegan <pg@bowt.ie>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>
Subject: Re: Index Skip Scan
Date: Sat, 22 Jun 2019 10:17:04 +0000
Message-ID: <1561198638439.56867@Optiver.com> (raw)
In-Reply-To: <a8b5c2d9-7b23-e12b-fc6b-eee63dc8dacf@redhat.com>
References: <2685f020-f302-9bf6-1b5c-ebc2bcebc635@2ndquadrant.com>
	<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>
	<1559362223126.18661@Optiver.com>
	<CA+FpmFeKh9BAhDTAf-ZWQ=pvVaRy+Lnahb_8=OOFqcVXJDBWWQ@mail.gmail.com>
	<988b7763-5c55-1be1-b7b6-f3a68eff0d40@redhat.com>
	<CA+q6zcVnRRqktw4cg2Vddy3S+VY3Tsmit0kausYtctvsTum7tA@mail.gmail.com>
	<1559763453006.85615@Optiver.com>
	<1559763570329.85227@Optiver.com>
	<4d469bf2-49ff-bc3d-a428-6c7b5d87d526@redhat.com>
	<CAKJS1f86FgODuUnHiQ25RKeuES4qTqeNxm1QbqJWrBoZxVGLiQ@mail.gmail.com>
	<CA+q6zcXoYjCtyU-jFU_=p2DL_K3BHbBEEbJHaJTxdvRX3cU-Sg@mail.gmail.com>
	<CA+q6zcVVigO_1KRBxvzKfnR593CEE7FKR-bwP-XcH6AVHOo4+g@mail.gmail.com>
	<a8b5c2d9-7b23-e12b-fc6b-eee63dc8dacf@redhat.com>

The following sql statement seems to have incorrect results - some logic in the backwards scan is currently not entirely right.

-Floris


drop table if exists a;
create table a (a int, b int, c int);
insert into a (select vs, ks, 10 from generate_series(1,5) vs, generate_series(1, 10000) ks);
create index on a (a,b);
analyze a;
select distinct on (a) a,b from a order by a desc, b desc;
explain select distinct on (a) a,b from a order by a desc, b desc;

DROP TABLE
CREATE TABLE
INSERT 0 50000
CREATE INDEX
ANALYZE
 a |   b   
---+-------
 5 | 10000
 5 |     1
 4 |     1
 3 |     1
 2 |     1
 1 |     1
(6 rows)

                                   QUERY PLAN                                    
---------------------------------------------------------------------------------
 Index Only Scan Backward using a_a_b_idx on a  (cost=0.29..1.45 rows=5 width=8)
   Scan mode: Skip scan
(2 rows)






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  Cc: florisvannee@Optiver.com, 9erthalion6@gmail.com, david.rowley@2ndquadrant.com, jtc331@gmail.com, jesper.pedersen@redhat.com, rafia.pghackers@gmail.com, jeff.janes@gmail.com, thomas.munro@gmail.com, pg@bowt.ie, tomas.vondra@2ndquadrant.com, thomas.munro@enterprisedb.com, a.korotkov@postgrespro.ru
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