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help / color / mirror / Atom feedFrom: Peter Geoghegan <[email protected]>
To: Robert Haas <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Georgios <[email protected]>
Cc: Thomas Munro <[email protected]>
Cc: Konstantin Knizhnik <[email protected]>
Cc: Dilip Kumar <[email protected]>
Subject: Re: index prefetching
Date: Mon, 11 Nov 2024 15:12:20 -0500
Message-ID: <CAH2-WznmrgwFShyKuKjF2v7M_Eid6VzKd+SRPjh4-y68T6uCDw@mail.gmail.com> (raw)
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On Mon, Nov 11, 2024 at 2:00 PM Peter Geoghegan <[email protected]> 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%4...
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.
--
Peter Geoghegan
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