From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Jesper Pedersen <jesper.pedersen@redhat.com>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: MDAM techniques and Index Skip Scan patch
Date: Mon, 28 Mar 2022 22:07:44 -0400
Message-ID: <1065129.1648519664@sss.pgh.pa.us> (raw)
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References: <CAH2-WzmUscvoxVkokHxP=uPTDjSi0tJkFpUPD-CeA35dvn-CMw@mail.gmail.com>
<2587523.1647982549@sss.pgh.pa.us>
<CAH2-Wz=yejXW1S=R8TOmOwuG8Cu64dQZyYCs1BGzc_0dLXqQMQ@mail.gmail.com>
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
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Subject: Re: MDAM techniques and Index Skip Scan patch
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