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.
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,