On Tue, Jun 21, 2016 at 10:59:25AM +1200, Thomas Munro wrote: > On Fri, Jun 17, 2016 at 3:36 PM, Noah Misch <n...@leadboat.com> wrote: > > I agree the non-atomic, unlogged change is a problem. A related threat > > doesn't require a torn page: > > > > AssignTransactionId() xid=123 > > heapam.c:3931 HeapTupleHeaderSetXmax(oldtup.t_data, 123); > > some ERROR before heap_update() finishes > > rollback; -- xid=123 > > some backend flushes the modified page > > immediate shutdown > > AssignTransactionId() xid=123 > > commit; -- xid=123 > > > > If nothing wrote an xlog record that witnesses xid 123, the cluster can > > reassign it after recovery. The failed update is now considered a > > successful > > update, and the row improperly becomes dead. That's important. > > I wonder if that was originally supposed to be handled with the > HEAP_XMAX_UNLOGGED flag which was removed in 11919160. A comment in > the heap WAL logging commit f2bfe8a2 said that tqual routines would > see the HEAP_XMAX_UNLOGGED flag in the event of a crash before logging > (though I'm not sure if the tqual routines ever actually did that).
HEAP_XMAX_UNLOGGED does appear to have originated in contemplation of this same hazard. Looking at the three commits in "git log -S HEAP_XMAX_UNLOGGED" (f2bfe8a b58c041 1191916), nothing ever completed the implementation by testing for that flag. -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers