| 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 15:00:10 |
| Message-ID: | CA+q6zcVVaWQvD91R0tkPoHsjY-u_utE2Y3aVuYNxKFMhcd8OdA@mail.gmail.com |
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| Lists: | pgsql-hackers |
> On Wed, Jul 10, 2019 at 4:52 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?
>
> create table a (a int, b int, c int);
> insert into a (select vs, ks, 10 from generate_series(1,5) vs, generate_series(1, 10000) ks);
> create index on a (a,b);
> analyze a;
>
> set enable_indexskipscan=1; // setting this to 0 yields different results
> set random_page_cost=1;
> explain SELECT DISTINCT ON (a) a,b FROM a;
>
> BEGIN;
> DECLARE c SCROLL CURSOR FOR SELECT DISTINCT ON (a) a,b FROM a;
>
> FETCH FROM c;
> FETCH BACKWARD FROM c;
>
> FETCH 6 FROM c;
> FETCH BACKWARD 6 FROM c;
>
> FETCH 6 FROM c;
> FETCH BACKWARD 6 FROM c;
>
> END;
Ok, give me a moment, I'll check.
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