Re: Index Skip Scan

From: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
To: Floris Van Nee <florisvannee(at)optiver(dot)com>
Cc: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, James Coleman <jtc331(at)gmail(dot)com>, Rafia Sabih <rafia(dot)pghackers(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Peter Geoghegan <pg(at)bowt(dot)ie>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Bhushan Uparkar <bhushan(dot)uparkar(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>
Subject: Re: Index Skip Scan
Date: 2019-07-10 15:05:49
Message-ID: CA+q6zcX-8KTkkpydVAKDryGU9MZhP-nW3U-cj2AS9r4_69wBrA@mail.gmail.com
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> On Wed, Jul 10, 2019 at 5:00 PM Floris Van Nee <florisvannee(at)optiver(dot)com> wrote:
>
>
> > 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.

Yeah, I'm aware. The last time when I was busy with cursors I've managed to
make it work as I wanted, so at that time I decided to keep it like that, even
though without skip scan it wasn't doing backwards.

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