BRIN cost estimate breaks geometric indexes

Hi,

Found out today that BRIN indexes don't really work for PostGIS and box
datatypes.

Since
https://github.com/postgres/postgres/commit/7e534adcdc70866e7be74d626b0ed067c890a251
Postgres
requires datatype to provide correlation statistics. Such statistics wasn't
provided by PostGIS and box types.

Today I tried to replace a 200gb gist index with 8mb brin index and queries
didn't work as expected - it was never used. set enable_seqscan=off helped
for a bit but that's not a permanent solution.
Plans for context:
https://gist.github.com/Komzpa/2cd396ec9b65e2c93341e9934d974826

Debugging session on #postgis IRC channel leads to this ticket to create a
(not that meaningful) correlation statistics for geometry datatype:
https://trac.osgeo.org/postgis/ticket/4625#ticket

Postgres Professional mentioned symptoms of the issue in their in-depth
manual: https://habr.com/ru/company/postgrespro/blog/346460/ - box datatype
showed same unusable BRIN symptoms for them.

A reasonable course of action on Postgres side seems to be to not assume
selectivity of 1 in absence of correlation statistics, but something that
would prefer such an index to a parallel seq scan, but higher than similar
GIST.

Any other ideas?

--
Darafei Praliaskouski
Support me: http://patreon.com/komzpa