On 25/10/2025 21:01, Joel Jacobson wrote:
On Sat, Oct 25, 2025, at 15:08, Arseniy Mukhin wrote:
I reimplemented the test in 0002 as an isolation test and added the
commit message. PFA the new version.
I've rebased the patches so they can be applied on top of v12:
- v12-vacuum_notify_queue_cleanup-without-code-dup.txt
- v12-vacuum_notify_queue_cleanup-with-code-dup.txt
I also want to share a new idea that Heikki Linnakangas came up with
during PgConf25 in Riga.
The idea is basically to scan the notification queue from tail to head,
and update xids that precedes newFrozenXid, to either
FrozenTransactionId if committed, or InvalidTransactionId if not.
I've implemented this by adding a new extern function
AsyncNotifyFreezeXids to async.h, called from vac_update_datfrozenxid.
This way, we don't need to hold back the advancement of newFrozenXid in
vac_update_datfrozenxid.
Attempt of implementing Heikki's idea:
- async_notify_freeze_xids.txt
Thanks!
This idea has the benefit of never holding back newFrozenXid,
however, I see some problems with it:
- If there is a bug in the code, we could accidentally overwrite
xid values we didn't intend to overwrite, and maybe we would never
find out that we did.
- We wouldn't know for sure that we've understood the cause of
this bug.
With v12-vacuum_notify_queue_cleanup, we instead have the downside of
possibly holding back newFrozenXid, but with the advantage of giving us
higher confidence in that we've correctly identified the cause of the
bug.
Meh. Robustness is good and all, and in heap tuples we don't overwrite
the xmin/xmax but just set a FROZEN flag, precisely so that we preserve
evidence if something goes wrong. But I can't get too worked up about
that for the async notification queue.
That said, we could add COMMITTED/INVALID hint bits to AsyncQueueEntry,
similar to heap tuples, and set the hint bit instead of replacing the
original xid. That might be good for performance too: If the first
backend to call TransactionIdDidCommit() on an AsyncQueueEntry would set
the hint bit, that would save the effort for other listening backends.
- Heikki
