Hi, On 2020-07-31 12:42:51 -0400, Robert Haas wrote: > On Fri, Jul 31, 2020 at 12:33 PM Andres Freund <and...@anarazel.de> wrote: > > I'm not sure what I was thinking "back then", but right now I'd argue > > that the best lock against vacuum isn't a SUE, but announcing the > > correct ->xmin, so you can be sure that clog entries won't be yanked out > > from under you. Potentially with the right flag sets to avoid old enough > > tuples eing pruned. > > Suppose we don't even do anything special in terms of advertising > xmin. What can go wrong? To have a problem, we've got to be running > concurrently with a vacuum that truncates clog. The clog truncation > must happen before our XID lookups, but vacuum has to remove the XIDs > from the heap before it can truncate. So we have to observe the XIDs > before vacuum removes them, but then vacuum has to truncate before we > look them up. But since we observe them and look them up while holding > a ShareLock on the buffer, this seems impossible. What's the flaw in > this argument?
The page could have been wrongly marked all-frozen. There could be interactions between heap and toast table that are checked. Other bugs could apply, like a broken hot chain or such. > If we do need to do something special in terms of advertising xmin, > how would you do it? Normally it happens by registering a snapshot, > but here all we would have is an XID; specifically, the value of > relfrozenxid that we observed. An appropriate procarray or snapmgr function would probably suffice? Greetings, Andres Freund
