Date: Thu, 5 Sep 2019 15:41:28 -0400
From: Alvaro Herrera from 2ndQuadrant <alvherre@alvh.no-ip.org>
To: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Floris Van Nee <florisvannee@optiver.com>,
	Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	"jesper.pedersen@redhat.com" <jesper.pedersen@redhat.com>,
	David Rowley <david.rowley@2ndquadrant.com>,
	Thomas Munro <thomas.munro@gmail.com>,
	James Coleman <jtc331@gmail.com>,
	Rafia Sabih <rafia.pghackers@gmail.com>,
	Jeff Janes <jeff.janes@gmail.com>, Peter Geoghegan <pg@bowt.ie>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	Thomas Munro <thomas.munro@enterprisedb.com>,
	Bhushan Uparkar <bhushan.uparkar@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>,
	Alexander Korotkov <a.korotkov@postgrespro.ru>
Subject: Re: Index Skip Scan
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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