Re: asynchronous and vectorized execution

On 2016-05-11 12:27:55 -0400, Robert Haas wrote:
> On Wed, May 11, 2016 at 11:49 AM, Andres Freund <andres(at)anarazel(dot)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?