Re: SKIP LOCKED DATA (work in progress)

Thomas Munro wrote:
> On 27 August 2014 17:18, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
> > Thomas Munro wrote:

> >> Yes it does, thanks Alvaro and Craig. I think the attached spec
> >> reproduces the problem using that trick, ie shows NOWAIT blocking,
> >> presumably in EvalPlanQualFetch (though I haven't stepped through it
> >> with a debugger yet). I'm afraid I'm out of Postgres hacking cycles
> >> for a few days, but next weekend I should have a new patch that fixes
> >> this by teaching EvalPlanQualFetch about wait policies, with isolation
> >> tests for NOWAIT and SKIP LOCKED.
> >
> > Hmm, http://www.postgresql.org/message-id/51FB6703.9090801@2ndquadrant.com
>
> Thanks, I hadn't seen this, I should have checked the archives better.
> I have actually already updated my patch to handle EvalPlanQualFetch
> with NOWAIT and SKIP LOCKED with isolation specs, see attached. I
> will compare with Craig's and see if I screwed anything up... of
> course I am happy to merge and submit a new patch on top of Craig's if
> it's going to be committed.

I tried Craig's patch with your test case and found that it stalls in
XactLockTableWait inside EPQFetch because it doesn't throw an error in
the noWait case before waiting. I think I will fix that and push,
including both test cases. Then we can see about rebasing your patch.

I am wondering about backpatching Craig's fix. It looks to me like it
should be backpatched as far back as NOWAIT exists, but that was in 8.1
and we haven't ever gotten a complaint until Craig's report AFAIK, which
I understand wasn't coming from a user finding a problem but rather some
new development. So I hesitate.

--
Álvaro Herrera http://www.2ndQuadrant.com/
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