On Wed, May 11, 2016 at 12:30 PM, Andres Freund <andres@anarazel.de> wrote:
> On 2016-05-11 12:27:55 -0400, Robert Haas wrote:
>> On Wed, May 11, 2016 at 11:49 AM, Andres Freund <andres@anarazel.de> wrote:
>> > On 2016-05-11 10:12:26 -0400, Robert Haas wrote:
>> >> > Hm. Do we really have to keep the page locked in the page-at-a-time
>> >> > mode? Shouldn't the pin suffice?
>> >>
>> >> I think we need a lock to examine MVCC visibility information. A pin
>> >> is enough to prevent a tuple from being removed, but not from having
>> >> its xmax and cmax overwritten at almost but not quite exactly the same
>> >> time.
>> >
>> > We already batch visibility lookups in page-at-a-time
>> > mode. Cf. heapgetpage() / scan->rs_vistuples. So we can evaluate quals
>> > after releasing the lock, but before the pin is released, without that
>> > much effort. IIRC that isn't used for index lookups, but that's
>> > probably a good idea.
>>
>> The trouble with that is that if you fail the qual, you have to relock
>> the page. Which kinda sucks, if the qual is really simple.
>
> Hm? I'm missing something here? We currently do the visibility checks in
> bulk for the whole page. After that we release the page lock. What
> prevents us from executing the quals directly after that? And why would
> you need to relock the page?
Oh, yeah, in page-at-a-time mode we can release the lock first. I was
thinking at what to do in tuple-at-a-time mode (i.e. when the page is
not all-visible).
--
Robert Haas
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