| From: | Dmitry Dolgov <9erthalion6(at)gmail(dot)com> |
|---|---|
| To: | Floris Van Nee <florisvannee(at)optiver(dot)com> |
| Cc: | Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(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>, James Coleman <jtc331(at)gmail(dot)com> |
| Subject: | Re: Index Skip Scan |
| Date: | 2019-06-01 16:57:31 |
| Message-ID: | CA+q6zcVJXRnsLXDK21knNnUpebWuTA-rpj5vHToA9Tyw5N+qaw@mail.gmail.com |
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| Lists: | pgsql-hackers |
> On Sat, Jun 1, 2019 at 5:34 PM Floris Van Nee <florisvannee(at)optiver(dot)com> wrote:
>
> I did a little bit of investigation and it seems to occur because in
> pathkeys.c the function pathkey_is_redundant considers pathkeys redundant if
> there is an equality condition with a constant in the corresponding WHERE
> clause.
> ...
> However, the index skip scan interprets this as that it has to skip over just
> the first column.
Right, passing correct number of columns fixes this particular problem. But
while debugging I've also discovered another related issue, when the current
implementation seems to have a few assumptions, that are not correct if we have
an index condition and a distinct column is not the first in the index. I'll
try to address these in a next version of the patch in the nearest future.
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