Re: Making all nbtree entries unique by having heap TIDs participate in comparisons

On Sun, Mar 10, 2019 at 12:53 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> Ah, yeah. Not sure. I wrote it as "searching_for_pivot_tuple" first, but
> changed to "searching_for_leaf_page" at the last minute. My thinking was
> that in the page-deletion case, you're trying to re-locate a particular
> leaf page. Otherwise, you're searching for matching tuples, not a
> particular page. Although during insertion, I guess you are also
> searching for a particular page, the page to insert to.

I prefer something like "searching_for_pivot_tuple", because it's
unambiguous. Okay with that?

> It's a hot codepath, but I doubt it's *that* hot that it matters,
> performance-wise...

I'll figure that out. Although I am currently looking into a
regression in workloads that fit in shared_buffers, that my
micro-benchmarks didn't catch initially. Indexes are still much
smaller, but we get a ~2% regression all the same. OTOH, we get a
7.5%+ increase in throughput when the workload is I/O bound, and
latency is generally no worse and even better with any workload.

I suspect that the nice top-down approach to nbtsplitloc.c has its
costs...will let you know more when I know more.

> > The idea with pg_upgrade'd v3 indexes is, as I said a while back, that
> > they too have a heap TID attribute. nbtsearch.c code is not allowed to
> > rely on its value, though, and must use
> > minusinfkey/searching_for_pivot_tuple semantics (relying on its value
> > being minus infinity is still relying on its value being something).
>
> Yeah. I find that's a complicated way to think about it. My mental model
> is that v4 indexes store heap TIDs, and every tuple is unique thanks to
> that. But on v3, we don't store heap TIDs, and duplicates are possible.

I'll try it that way, then.

--
Peter Geoghegan