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

On Wed, Nov 13, 2024 at 3:30 PM Dmitry Dolgov <9erthalion6(at)gmail(dot)com> wrote:
> > 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
> > 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");

> 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.

It's a sample dataset that I've found useful from time to time,
particularly when testing nbtree features. Usually using a composite
index like the one I described.

One slightly useful (though far from unique) property of such an index
is that it contains data that's low cardinality (sometimes extremely
low cardinality) across multiple index columns. With variable-width
(text) index columns. That specific combination made the index a
decent test of certain issues affecting the nbtsplitloc.c split point
choice logic during work on Postgres 12 and 13. I believe that
Matthias independently found it useful on a number of other occasions,
too.

See https://wiki.postgresql.org/wiki/Sample_Databases for instructions
on how to set it up for yourself. You could probably come up with a
way of generating a similar dataset, without needing to download
anything, though. The fact that I found it useful in the past is at
least somewhat arbitrary.

--
Peter Geoghegan