Re: The case for removing replacement selection sort

On Fri, Jul 14, 2017 at 6:20 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> With the additional enhancements made to Postgres 10, I doubt that
> there are any remaining cases where it wins.

I tried my favorite sorting test case -- pgbench -i -s 300 and then
reindex index pgbench_accounts_pkey. I set maintenance_work_mem to
4MB so that replacement selection would be chosen.

On master, this takes ~21.5 seconds; with replacement_sort_tuples = 0,
it takes ~19.1 seconds. So, disabling replacement selection is a win.

On 9.6, this takes ~19.1 seconds; with replacement_sort_tuples = 0, it
takes ~23 seconds. So, disabling replacement selection is a loss.

This supports your theory that we should go ahead and remove
replacement selection. It does however both me a bit that this test
case is apparently slower in master than in 9.6, and not just with
very small values of work_mem. With default maintenance_work_mem
(64MB), this takes about 13 seconds on 9.6 but about 15 seconds on
master -- and with that setting replacement selection is not chosen at
all.

Any idea what causes this regression?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company