On Thu, Nov 2, 2017 at 10:26 PM, Peter Geoghegan <p...@bowt.ie> wrote: > On Thu, Nov 2, 2017 at 9:44 AM, Robert Haas <robertmh...@gmail.com> wrote: >> The second commit (22576734b805fb0952f9be841ca8f643694ee868) is where >> I think things get a lot more dangerous. The problem (as Andres >> pointed out to me this afternoon) is that it seems possible to end up >> with a situation where there should be two HOT chains on a page, and >> because of the weakened xmin/xmax checking rules, we end up thinking >> that the second one is a continuation of the first one, which will be >> all kinds of bad news. That would be particularly likely to happen in >> releases from before we invented HEAP_XMIN_FROZEN, when there's no >> xmin/xmax matching at all, but could happen on later releases if we >> are extraordinarily unlucky (i.e. xmin of the first tuple in the >> second chain happens to be the same as the pre-freeze xmax in the old >> chain, probably because the same XID was used to update the page in >> two consecutive epochs). Fortunately, that commit is (I think) not >> released anywhere. > > FWIW, if you look at the second commit > (22576734b805fb0952f9be841ca8f643694ee868) carefully, you'll realize > that it doesn't even treat those two cases differently. It was buggy > even on its own terms. The FrozenTransactionId test used an xmin from > HeapTupleHeaderGetXmin(), not HeapTupleHeaderGetRawXmin().
Oh, wow. You seem to be correct. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-committers mailing list (pgsql-committers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-committers
