Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On 2016-06-16 12:02:39 -0400, Robert Haas wrote:
> On Thu, Jun 16, 2016 at 11:37 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> If that were really true, why would we not have the problem in
> >> current production versions that the toast table could be vacuumed
> >> before the heap, leading to exactly the issue you are talking
> >> about?
> >
> > The issue isn't there without the feature, because we (should) never
> > access a tuple/detoast a column when it's invisible enough for the
> > corresponding toast tuple to be vacuumed away. But with
> > old_snapshot_timeout that's obviously (intentionally) not the case
> > anymore. Due to old_snapshot_threshold we'll prune tuples which,
> > without it, would still be considered HEAPTUPLE_RECENTLY_DEAD.
>
> Is there really an assumption that the heap and the TOAST heap are
> only ever vacuumed with the same OldestXmin value? Because that seems
> like it would be massively flaky.

There's not. They can be vacuumed days apart. But if we vacuum the toast
table with an OldestXmin, and encounter a dead toast tuple, by the
definition of OldestXmin (excluding STO), there cannot be a session
reading the referencing tuple anymore - so that shouldn't matter.

IIRC we actually reverted a patch that caused significant problems
around this. I think there's a small race condition around
ProcessStandbyHSFeedbackMessage(), and you can restart with a different
vacuum_defer_cleanup_age (we should just remove that), but other than
that we shouldn't run into any issues without STO.