Re: REPACK (CONCURRENTLY) can silently lose updates when the toast table is rewritten

On Wed, Sep 23, 2026 at 9:23 AM Antonin Houska <ah(at)cybertec(dot)at> wrote:
>
> shihao zhong <zhong950419(at)gmail(dot)com> wrote:
>
> > > or whether the relfilenode should be re-checked after the snapshot is built
> >
> > Holding the toast lock from the start deadlocks. A session that asks for
> > AccessExclusiveLock gets an XID before it waits, and the decoding worker
> > waits for all XIDs while it sets up.
>
> The same (supposedly low) deadlock risk already exists for the main table, see
> this comment in rebuild_relation():
>
> /*
> * Start the worker that decodes data changes applied while we're
> * copying the table contents.
> *
> * Note that the worker has to wait for all transactions with XID
> * already assigned to finish. If some of those transactions is
> * waiting for a lock conflicting with ShareUpdateExclusiveLock on our
> * table (e.g. it runs CREATE INDEX), we can end up in a deadlock.
> * Not sure this risk is worth unlocking/locking the table (and its
> * clustering index) and checking again if it's still eligible for
> * REPACK CONCURRENTLY.
> */
> start_repack_decoding_worker(tableOid);
>
> I'm not sure if locking the TOAST relation earlier would make the situation
> worse.

Agreed.

So I think the simplest fix would be to acquire a lock on the TOAST
table before starting the repack worker. It would make the case in
question fail with a deadlock, instead of silently losing updates.

The proposed patch also fixes the problem, but I'm concerned that it
repeatedly starts and stops the repack worker without any limit. I
think we could error out if we detect a concurrent rewrite, so that
users can re-run REPACK CONCURRENTLY. This check could also be done on
the repack worker side: after getting the relfilelocator of the TOAST
table and initializing the logical decoding, the repack worker
rechecks the relfilelocator. If they don't match, it raises an error.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com