Re: MDAM techniques and Index Skip Scan patch

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Peter Geoghegan <pg(at)bowt(dot)ie>
Cc: Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: MDAM techniques and Index Skip Scan patch
Date: 2022-03-29 02:07:44
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Peter Geoghegan <pg(at)bowt(dot)ie> writes:
> The terminology in this area is a mess. MySQL calls
> SELECT-DISTINCT-that-matches-an-index "loose index scans". I think
> that you're talking about skip scan when you say "loose index scan".
> Skip scan is where there is an omitted prefix of columns in the SQL
> query -- omitted columns after the first column that lack an equality
> qual.

Right, that's the case I had in mind --- apologies if my terminology
was faulty. btree can actually handle such a case now, but what it
fails to do is re-descend from the tree root instead of plowing
forward in the index to find the next matching entry.

> It might be useful for somebody to go write a "taxonomy of MDAM
> techniques", or a glossary.

+1. We at least need to be sure we all are using these terms
the same way.

regards, tom lane

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