| From: | Alvaro Herrera from 2ndQuadrant <alvherre(at)alvh(dot)no-ip(dot)org> |
|---|---|
| To: | Dmitry Dolgov <9erthalion6(at)gmail(dot)com> |
| Cc: | Floris Van Nee <florisvannee(at)optiver(dot)com>, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, "jesper(dot)pedersen(at)redhat(dot)com" <jesper(dot)pedersen(at)redhat(dot)com>, David Rowley <david(dot)rowley(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>, 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: | 2019-09-05 19:41:28 |
| Message-ID: | 20190905194128.GA10752@alvherre.pgsql |
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| Lists: | pgsql-hackers |
On 2019-Sep-05, Dmitry Dolgov wrote:
> Here is the version in which stepping between the pages works better. It seems
> sufficient to fix the case you've mentioned before, but for that we need to
> propagate keepPrev logic through `_bt_steppage` & `_bt_readnextpage`, and I
> can't say I like this solution. I have an idea that maybe it would be simpler
> to teach the code after index_skip to not do `_bt_next` right after one skip
> happened before. It should immediately elliminate several hacks from index skip
> itself, so I'll try to pursue this idea.
Cool.
I think multiplying two ScanDirections to watch for a negative result is
pretty ugly:
/*
* If advancing direction is different from index direction, we must
* skip right away, but _bt_skip requires a starting point.
*/
if (direction * indexscan->indexorderdir < 0 &&
!node->iss_FirstTupleEmitted)
Surely there's a better way to code that?
I think "scanstart" needs more documentation, both in the SGML docs as
well as the code comments surrounding it.
Please disregard my earlier comment about FirstTupleEmitted. I was
thinking that index_skip would itself emit a tuple (ie. call some
"getnext" internally) rather than just repositioning. There might still
be some more convenient way to represent this, but I have no immediate
advice.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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