Re: btree: implement dynamic prefix truncation (was: Improving btree performance through specializing by key shape, take 2)

> On Tue, Aug 13, 2024 at 02:39:10PM GMT, Peter Geoghegan wrote:
> On Tue, Aug 6, 2024 at 5:42 PM Matthias van de Meent
> <boekewurm+postgres@gmail.com> wrote:
> > Attached is version 16 now.
>
> I ran this with my old land registry benchmark, used for validating
> the space utilization impact of nbtree deduplication (among other
> things). This isn't obviously the best benchmark for this sort of
> thing, but I seem to recall that you'd used it yourself at some point.
> To validate work in this area, likely including this patch. So I
> decided to start there.
>
> To be clear, this test involves bulk loading of an unlogged table (the
> land registry table). The following composite index is created on the
> table before we insert any rows, so most of the cycles here are in
> index maintenance including _bt_search descents:
>
> CREATE INDEX composite ON land2 USING btree (county COLLATE "C", city
> COLLATE "C", locality COLLATE "C");
>
> I wasn't able to see much of an improvement with this patch applied.
> It went from  ~00:51.598 to ~00:51.053. That's a little disappointing,
> given that this is supposed to be a sympathetic case for the patch.
> Can you suggest something else? (Granted, I understand that this patch
> has some complicated relationship with other patches of yours, which I
> don't understand currently.)

Under the danger of showing my ignorance, what is the definition of land
registry benchmark? I think it would be useful if others could reproduce
the results as well, especially if they're somewhat surprising.

At the same time I would like to note, that dynamic prefix truncation
can be useful not only on realistic data, but in some weird-but-useful
cases, e.g. for partitioned B-Tree with an artificial leading key
separating partitions. It's a hypothetical case, which can't justify
a feature of course, but makes it worth investigating.