| From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
|---|---|
| To: | Floris Van Nee <florisvannee(at)optiver(dot)com> |
| Cc: | Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(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>, 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: | 2020-01-22 22:23:36 |
| Message-ID: | CAH2-Wzm1YNybYKaDX=dR+Oq8TubhXqb+Q0mhOMEyVAg=k5P=Aw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Jan 22, 2020 at 1:09 PM Floris Van Nee <florisvannee(at)optiver(dot)com> wrote:
> The loose index scan shouldn't return a tuple twice. It should only be able to skip 'further', so that shouldn't be a problem. Out of curiosity, why can't index scans return the same tuple twice? Is there something in the executor that isn't able to handle this?
I have no reason to believe that the executor has a problem with index
scans that return a tuple more than once, aside from the very obvious:
in general, that will often be wrong. It might not be wrong when the
scan happens to be input to a unique node anyway, or something like
that.
I'm not particularly concerned about it. Just wanted to be clear on
our assumptions for loose index scans -- if loose index scans were
allowed to return a tuple more than once, then that would at least
have to at least be considered in the wider context of the executor
(but apparently they're not, so no need to worry about it). This may
have been mentioned somewhere already. If it is then I must have
missed it.
--
Peter Geoghegan
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