From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@bowt.ie>
cc: Jesper Pedersen <jesper.pedersen@redhat.com>,
        Dmitry Dolgov <9erthalion6@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: MDAM techniques and Index Skip Scan patch
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Comments: In-reply-to Peter Geoghegan <pg@bowt.ie>
	message dated "Mon, 28 Mar 2022 18:58:22 -0700"
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Date: Mon, 28 Mar 2022 22:07:44 -0400
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Peter Geoghegan <pg@bowt.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