Re: snapshot too old issues, first around wraparound and then more.

On Wed, Apr 1, 2020 at 3:43 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> The thing that makes me really worried is that the contents of the time
> mapping seem very wrong. I've reproduced query results in a REPEATABLE
> READ transaction changing (pruned without triggering an error). And I've
> reproduced rows not getting removed for much longer than than they
> should, according to old_snapshot_threshold.

I think it would be a good idea to add a system view that shows the
contents of the mapping. We could make it a contrib module, if you
like, so that it can even be installed on back branches. We'd need to
move the structure definition from snapmgr.c to a header file, but
that doesn't seem like such a big deal.

Maybe that contrib module could even have some functions to simulate
aging without the passage of any real time. Like, say you have a
function or procedure old_snapshot_pretend_time_has_passed(integer),
and it moves oldSnapshotControl->head_timestamp backwards by that
amount. Maybe that would require updating some other fields in
oldSnapshotControl too but it doesn't seem like we'd need to do a
whole lot.

--
Robert Haas
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