On Mon, Nov 11, 2024 at 2:00 PM Peter Geoghegan <p...@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. -- Peter Geoghegan
