Re: Adding REPACK [concurrently]

Hi,

On 2026-04-08 14:05:49 +0530, Amit Kapila wrote:
> On Wed, Apr 8, 2026 at 1:54 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2026-04-07 00:22:32 +0200, Alvaro Herrera wrote:
> > > From 4303eea0a72408183f9f5afcf8d2801df20f8ffe Mon Sep 17 00:00:00 2001
> > > From: Antonin Houska <ah(at)cybertec(dot)at>
> > > Date: Wed, 1 Apr 2026 17:35:47 +0200
> > > Subject: [PATCH v56 3/3] Error out any process that would block at REPACK
> > >
> > > Any process waiting on REPACK to release its lock would actually cause
> > > it to deadlock when it tries to upgrade its lock to AEL, losing all work
> > > done to that point. We avoid this by teaching the deadlock detector to
> > > raise an error when this condition is detected.
> >
> > I'm rather doubtful that that is ok.
> >
>
> Another possible idea is that after copying table_data to the new
> table, we mark the old table as in_use_by_repack and release the
> ShareUpdateExclusiveLock on the old table. Then the function
> CheckTableNotInUse() should be updated to give an ERROR if the table
> is marked as in_use_by_repack. Now, acquiring AEL by repack
> (concurrently) should be safe because all concurrent DDLs should be
> errored out due to flag in_use_by_repack. Can this address the problem
> we are worried about the lock upgrade?

I don't think this is a viable path. You need to prevent any further lock
acquisitions on the relation to be able to swap it, not just conflicting DDL.
And you need to wait for all pre-existing locks to have been released. That
doesn't really get easier by what you propose.

I don't think CheckTableNotInUse() would work anyway - don't we already hold
locks by the point we call it? And even if that were not the case, there are
several paths to locking relations that don't ever go anywhere near
CheckTableNotInUse().

Greetings,

Andres Freund