On 2012-11-30 18:09:49 +0530, Pavan Deolasee wrote:
> On Fri, Nov 30, 2012 at 5:28 PM, Andres Freund <
andres@2ndquadrant.com>wrote:
>
> >
> > > >
> > > That seems to be safe to me. Anything thats been read above can't really
> > > change. The tuple is already locked, so a concurrent update/delete has to
> > > wait on us. We have a pin on the buffer, so VACUUM or HOT-prune can't
> > > happen either. I can't see any other operation that can really change
> > those
> > > fields.
> >
> > We only get the pin right there, I don't see any preexisting pin. Which
> > means we might have just opened a page thats in the process of being
> > pruned/vacuumed by another backend.
> >
>
> Hmm. Yeah, you're right. That is a possible risky scenario. Even though
> cleanup lock waits for all pins to go away, it will work only if every
> reader takes at least a SHARE lock unless it was continuously holding a pin
> on a buffer (in which case its OK to drop lock and read a tuple body
> without reacquiring it again). Otherwise, as you rightly pointed out, we
> could actually be reading a page which being actively cleaned up and tuples
> are being moved around.
