On Mon, Jun 24, 2024 at 4:36 PM Robert Haas <robertmh...@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
