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

On Mon, Jun 24, 2024 at 4:36 PM Robert Haas <robertmhaas@gmail.com> wrote:
> 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.

Your characterizing of OldestXmin + vistest as two horizons seems
pretty arbitrary to me. I know what you mean, of course, but it seems
like a distinction without a difference.

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

It definitely can do that. Just not in a way that meaningfully
increases the number of heap tuples that we can recognize as DEAD and
remove. At least not currently.

> 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?

That's my understanding, yes. That is, vistest is approximately the
same thing as OldestXmin anyway. At least for now.

--
Peter Geoghegan