| From: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
|---|---|
| To: | Dmitry Dolgov <9erthalion6(at)gmail(dot)com> |
| Cc: | Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, James Coleman <jtc331(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>, Peter Geoghegan <pg(at)bowt(dot)ie>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Bhushan Uparkar <bhushan(dot)uparkar(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru> |
| Subject: | Re: Index Skip Scan |
| Date: | 2019-02-28 21:45:15 |
| Message-ID: | CAMkU=1x0984uC7PuupnnF6rVeMg6q3OwhU8k906mVZODstaGgQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Feb 20, 2019 at 11:33 AM Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
wrote:
> > On Fri, Feb 1, 2019 at 8:24 PM Jesper Pedersen <
> jesper(dot)pedersen(at)redhat(dot)com> wrote:
> >
> > Dmitry and I will look at this and take it into account for the next
> > version.
>
> In the meantime, just to not forget, I'm going to post another version
> with a
> fix for cursor fetch backwards, which was crashing before.
This version of the patch can return the wrong answer.
create index on pgbench_accounts (bid, aid);
begin; declare c cursor for select distinct on (bid) bid, aid from
pgbench_accounts order by bid, aid;
fetch 2 from c;
bid | aid
-----+---------
1 | 1
2 | 100,001
fetch backward 1 from c;
bid | aid
-----+---------
1 | 100,000
Without the patch, instead of getting a wrong answer, I get an error:
ERROR: cursor can only scan forward
HINT: Declare it with SCROLL option to enable backward scan.
If I add "SCROLL", then I do get the right answer with the patch.
Cheers,
Jeff
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