On Fri, Nov 30, 2012 at 5:28 PM, Andres Freund <and...@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. > I think a concurrent heap_(insert|update)/PageAddItem might actually be > already dangerous because it might move line pointers around > > I don't we move the line pointers around ever because the indexes will be pointing to them. But the vacuum/prune is dangerous enough to require a SHARE lock here in any case. Thanks, Pavan -- Pavan Deolasee http://www.linkedin.com/in/pavandeolasee
