On 2014-03-17 16:17:35 -0300, Alvaro Herrera wrote: > Andres Freund wrote: > > On 2014-03-17 14:01:03 -0400, Tom Lane wrote: > > > Andres Freund <and...@2ndquadrant.com> writes: > > > > * I wonder if we should make the possible origins a bit more > > > > general as it's perfectly possible to trigger the problem without > > > > foreign keys. Maybe: "can arise when a table row that has been updated > > > > is row locked; that can e.g. happen when foreign keys are used." > > > > > > IIUC, this case only occurs when using the new-in-9.3 types of > > > nonexclusive row locks. I'm willing to bet that the number of > > > applications using those is negligible; so I think it's all right to not > > > mention that case explicitly, as long as the wording doesn't say that > > > foreign keys are the *only* cause (which I didn't). > > > > I actually think the issue could also occur with row locks of other > > severities (is that the correct term?). Alvaro probably knows better, > > but if I see correctly it's also triggerable if a backend waits for an > > updating transaction to finish and follow_updates = true is passed to > > heap_lock_tuple(). Which e.g. nodeLockRows.c does... > > Uhm. But at the bottom of that block, right above the "failed:" label > (heapam.c line 4527 in current master), we recheck the tuple for > "locked-only-ness"; and fail the whole operation by returning > HeapTupleUpdated, if it's not locked-only, no? Which would cause > ExecLockRows to grab the next version via EvalPlanQualFetch. > Essentially that check is a lock-conflict test, and the only thing that > does not conflict with an update is a FOR KEY SHARE lock.
What I was thinking of is the case where heap_lock_tuple() notices it needs to sleep and then in the if (require_sleep) block does a lock_updated_tuple(). If the updating transaction aborts while waiting lock_updated_tuple_rec() will issue a XLOG_HEAP2_LOCK_UPDATED for that row and then return MayBeUpdated. Which will make heap_lock_tuple() successfully lock the row, thereby resetting t_ctid during replay. What I missed is that case resetting the ctid chain is perfectly fine, since the pointed to tuple is actually dead. I was just confused by the fact that we do actually issue a XLOG_HEAP2_LOCK_UPDATED for a dead row. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers