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From: Peter Geoghegan <pg@bowt.ie>
Date: Mon, 11 Nov 2024 15:12:20 -0500
Message-ID: <CAH2-WznmrgwFShyKuKjF2v7M_Eid6VzKd+SRPjh4-y68T6uCDw@mail.gmail.com>
Subject: Re: index prefetching
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tomas Vondra <tomas@vondra.me>, Andres Freund <andres@anarazel.de>, 
	Melanie Plageman <melanieplageman@gmail.com>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Georgios <gkokolatos@protonmail.com>, 
	Thomas Munro <thomas.munro@gmail.com>, Konstantin Knizhnik <knizhnik@garret.ru>, 
	Dilip Kumar <dilipbalaut@gmail.com>
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On Mon, Nov 11, 2024 at 2:00=E2=80=AFPM Peter Geoghegan <pg@bowt.ie> wrote:
> The real sign that what I said is generally true of index AMs is that
> you'll see so few calls to
> LockBufferForCleanup/ConditionalLockBufferForCleanup. Only hash calls
> ConditionalLockBufferForCleanup at all (which I find a bit weird).
> Both GiST and SP-GiST call neither functions -- even during VACUUM. So
> GiST and SP-GiST make clear that index AMs (that support only MVCC
> snapshot scans) can easily get by without any use of cleanup locks
> (and with no externally significant use of buffer pins).

Actually, I'm pretty sure that it's wrong for GiST VACUUM to not
acquire a full cleanup lock (which used to be called a super-exclusive
lock in index AM contexts), as I went into some years ago:

https://www.postgresql.org/message-id/flat/CAH2-Wz%3DPqOziyRSrnN5jAtfXWXY7-=
BJcHz9S355LH8Dt%3D5qxWQ%40mail.gmail.com

I plan on playing around with injection points soon. I might try my
hand at proving that GiST VACUUM needs to do more here to avoid
breaking concurrent GiST index-only scans.

Issues such as this are why I place so much emphasis on formalizing
all the rules around TID recycling and dropping pins with index scans.
I think that we're still a bit sloppy about things in this area.

--=20
Peter Geoghegan