Re: Do we need so many hint bits?

On Mon, 2012-11-19 at 14:46 -0500, Tom Lane wrote:
> Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> > As I said elsewhere in the thread, I'm not planning to introduce any
> > additional locking. There is already precedent in IndexOnlyNext.
>
> Of course, that just begs the question of whether the code in
> IndexOnlyNext is correct. And after looking at it, it seems pretty
> broken, or at least the argument for its correctness is broken.
> That comment seems to consider only the case where the target tuple has
> just been inserted --- but what of the case where the target tuple is in
> process of being deleted? The deleter will not make a new index entry
> for that. Of course, there's a pretty long code path from marking a
> tuple deleted to committing the deletion, and it seems safe to assume
> that the deleter will hit a write barrier in that path. But I don't
> believe the argument here that the reader must hit a read barrier in
> between.

After digging in a little, this is bothering me now, too. I think it's
correct, and I agree with your analysis, but it seems a little complex
to explain why it works.

It also seems like a fortunate coincidence that we can detect clearing
the bit due to an insert, which we need to know about immediately; but
not detect a delete, which we don't care about until it commits.

I will try to update the comment along with my submission.

Regards,
Jeff Davis