Re: Vacuum ERRORs out considering freezing dead tuples from before OldestXmin

On Mon, Jun 24, 2024 at 3:23 PM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
> Are you more concerned about having a single horizon for pruning or
> about having a horizon that does not move backwards after being
> established at the beginning of vacuuming the relation?

I'm not sure I understand. The most important thing here is fixing the
bug. But if we have a choice of how to fix the bug, I'd prefer to do
it by having the pruning code test one horizon that is always correct,
rather than (as I think the patch does) having it test against two
horizons because as a way of covering possible discrepancies between
those values.

> Right now, in master, we do use a single horizon when determining what
> is pruned -- that from GlobalVisState. OldestXmin is only used for
> freezing and full page visibility determinations. Using a different
> horizon for pruning by vacuum than freezing is what is causing the
> error on master.

Agreed.

> I had always thought it was because the vacuuming backend's
> GlobalVisState will get updated periodically throughout vacuum and so,
> assuming the oldest running transaction changes, our horizon for
> vacuum would change. But, in writing this repro, it is actually quite
> hard to get GlobalVisState to update. Our backend's RecentXmin needs
> to have changed. And there aren't very many places where we take a new
> snapshot after starting to vacuum a relation. One of those is at the
> end of index vacuuming, but that can only affect the pruning horizon
> if we have to do multiple rounds of index vacuuming. Is that really
> the case we are thinking of when we say we want the pruning horizon to
> move forward during vacuum?

I thought the idea was that the GlobalVisTest stuff would force a
recalculation now and then, but maybe it doesn't actually do that?

Suppose process A begins a transaction, acquires an XID, and then goes
idle. Process B now begins a giant vacuum. At some point in the middle
of the vacuum, A ends the transaction. Are you saying that B's
GlobalVisTest never really notices that this has happened?

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