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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: Jesper Pedersen <jesper.pedersen@redhat.com>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: James Coleman <jtc331@gmail.com>
Cc: Rafia Sabih <rafia.pghackers@gmail.com>
Cc: Jeff Janes <jeff.janes@gmail.com>
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>
Subject: Re: Index Skip Scan
Date: Wed, 10 Jul 2019 15:00:23 +0000
Message-ID: <1562770824851.93603@Optiver.com> (raw)
In-Reply-To: <1562770342222.94509@Optiver.com>
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> Thanks for testing! Could you provide a test case to show what exactly is the
> problem?

Note that in the case of a regular non-skip scan, this cursor backwards works because the Unique node on top does not support backwards scanning at all. Therefore, when creating the cursor, the actual plan actually contains a Materialize node on top of the Unique+Index Scan nodes. The 'fetch backwards' never reaches the the index scan therefore, as it just fetches stuff from the materialize node.

-Floris




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