Andres Freund wrote:
> On 2014-03-17 14:01:03 -0400, Tom Lane wrote:
> > Andres Freund <andres@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
