Feedback-ID: id4a34324:Fastmail
Date: Thu, 9 Apr 2026 10:20:18 -0400
From: Andres Freund <andres@anarazel.de>
To: Antonin Houska <ah@cybertec.at>
Cc: Amit Kapila <amit.kapila16@gmail.com>,
	Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
 Alvaro Herrera <alvherre@alvh.no-ip.org>,
	Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Pg Hackers <pgsql-hackers@lists.postgresql.org>,
 Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
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Hi,

On 2026-04-09 10:43:14 +0200, Antonin Houska wrote:
> What Andres proposed (AFAIU) should help to avoid this problem because
> REPACK's request for AEL would get in front of the VACUUM's request for SUEL
> in the queue.

Note that that already happens today.

This works today (without the error triggering patch):

S1: REPACK starts
S2: LOCK TABLE / VACUUM / ... starts waiting
S1: REPACK tries to get AEL
S1: REPACK's lock requests get reordered in the wait queue to be before S2 and
    just gets the lock
S1: REPACK finishes
S2: lock acquisition completes.

That's because we do already have this "jumping the wait queue" logic, which I
had forgotten about.


What does *not* work is this:

S1: REPACK starts
S2: BEGIN; SELECT 1 FROM table LIMIT 1;
S2: LOCK TABLE / VACUUM / ... starts waiting
S1: REPACK tries to get AEL
S1: lock is not granted, can't be reordered to be before S2, because S2 holds
    conflicting lock, deadlock detector triggers
S2: lock acquisition completes

But with my proposal to properly teach the deadlock detector about assuming
there's a wait edge for the eventual lock upgrade by S1, the first example
would still work, because the lock upgrade would not be considered a hard
cycle, and the second example would have S2 error out.


> Anti-wraparound (failsafe) VACUUM is a bit different case [1] (i.e. it should
> possibly have higher priority than REPACK), but I think this prioritization
> should be implemented in other way than just letting it get in the way of
> REPACK (at the time REPACK is nearly finished).

Yea, it makes no sense to interrupt the long running repack, given that the
new relation will have much less stuff for vacuum to do.

Greetings,

Andres Freund