On Fri, Jun 17, 2016 at 3:36 PM, Noah Misch <noah@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).
--
Thomas Munro
http://www.enterprisedb.com
