Re: relfrozenxid may disagree with row XIDs after 1ccc1e05ae

On Fri, Mar 22, 2024 at 8:22 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Thu, Mar 21, 2024 at 1:22 PM Matthias van de Meent
> <boekewurm+postgres(at)gmail(dot)com> wrote:
> > > So it seems like Matthias, Peter, and Andres all agree that
> > > GlobalVisState->maybe_needed going backward is bad and causes this
> > > problem. Unfortunately, I don't understand the mechanism.
> >
> > There are 2 mechanisms I know of which allow this value to go backwards:
>
> I actually wasn't asking about the mechanism by which
> GlobalVisState->maybe_needed could go backwards. I was asking about
> the mechanism by which that could cause bad things to happen.
>
> > 1. Replication slots that connect may set their backend's xmin to an
> > xmin < GlobalXmin.
> > This is known and has been documented, and was considered OK when this
> > was discussed on the list previously.
>
> Right, OK.
>
> > 2. The commit abort path has a short window in which the backend's
> > xmin is unset and does not mirror the xmin of registered snapshots.
> > This is what I described in [0], and may be the worst (?) offender.
> >
> > [0] https://www.postgresql.org/message-id/CAEze2Wj%2BV0kTx86xB_YbyaqTr5hnE_igdWAwuhSyjXBYscf5-Q%40mail.gmail.com
>
> So, what I would say is that this sounds inadvertent and so perhaps we
> should do something about it, but also, it seems wrong to me that it
> causes any serious problem. As far as I know, we've always treated the
> result of an xmin calculation going backward as a rare but expected
> case with which everything that depends on xmin calculations must
> cope.

I'm still catching up here, so forgive me if this is a dumb question:
Does using GlobalVisState instead of VacuumCutoffs->OldestXmin when
freezing and determining relfrozenxid not solve the problem?

- Melanie