Re: why there is not VACUUM FULL CONCURRENTLY?

Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:

>
>
> > From bf2ec8c5d753de340140839f1b061044ec4c1149 Mon Sep 17 00:00:00 2001
> > From: Antonin Houska <ah(at)cybertec(dot)at>
> > Date: Mon, 13 Jan 2025 14:29:54 +0100
> > Subject: [PATCH 4/8] Add CONCURRENTLY option to both VACUUM FULL and CLUSTER
> > commands.
>
> > @@ -950,8 +1412,46 @@ copy_table_data(Relation NewHeap, Relation OldHeap, Relation OldIndex, bool verb
>
> > + if (concurrent)
> > + {
> > + PgBackendProgress progress;
> > +
> > + /*
> > + * Command progress reporting gets terminated at subtransaction
> > + * end. Save the status so it can be eventually restored.
> > + */
> > + memcpy(&progress, &MyBEEntry->st_progress,
> > + sizeof(PgBackendProgress));
> > +
> > + /* Release the locks by aborting the subtransaction. */
> > + RollbackAndReleaseCurrentSubTransaction();
> > +
> > + /* Restore the progress reporting status. */
> > + pgstat_progress_restore_state(&progress);
> > +
> > + CurrentResourceOwner = oldowner;
> > + }
>
> I was looking at 0002 to see if it'd make sense to commit it ahead of a
> fuller review of the rest, and I find that the reason for that patch is
> this hunk you have here in copy_table_data -- you want to avoid a
> subtransaction abort (which you use to release planner lock) clobbering
> the status. I think this a bad idea. It might be better to handle this
> in a different way, for instance
>
> 1) maybe have a flag that says "do not reset progress status during
> subtransaction abort"; REPACK would set that flag, so it'd be able to
> continue its business without having to memcpy the current status (which
> seems like quite a hack) or restoring it afterwards.
>
> 2) maybe subtransaction abort is not the best way to release the
> planning locks anyway. I think it might be better to have a
> ResourceOwner that owns those locks, and we do ResourceOwnerRelease()
> which would release them. I think this would be a novel usage of
> ResourceOwner so it needs more research. But if this works, then we
> don't need the subtransaction at all, and therefore we don't need
> backend progress restore at all either.

If this needs change, I prefer 2) because it's less invasive: 1) still affects
the progress monitoring code. I'll look at it.

--
Antonin Houska
Web: https://www.cybertec-postgresql.com