Re: relfrozenxid may disagree with row XIDs after 1ccc1e05ae

On Thu, Mar 21, 2024 at 1:22 PM Matthias van de Meent
<boekewurm+postgres@gmail.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.

--
Robert Haas
EDB: http://www.enterprisedb.com