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. Note the only way to pass that test is that either the tuple is locked-only (spelled in three different ways), or "!require_sleep". Am I completely misunderstanding what's being said here? -- Álvaro Herrera http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
