How do we support FULL JOIN on PostGIS types?

Hi!

Greetings from OSGeo Code sprint in Minneapolis :)

We're trying to make FULL JOIN on equality of geometry and can't figure out
why it doesn't work.

Here's reproducer, it works on bytea but not on PostGIS geometry throwing
out

ERROR: FULL JOIN is only supported with merge-joinable or hash-joinable
join conditions

https://trac.osgeo.org/postgis/ticket/4394

We already have a btree opclass with equality:
https://github.com/postgis/postgis/blob/svn-trunk/postgis/postgis.sql.in#L420

We also have hash equality opclass:
https://github.com/postgis/postgis/blob/svn-trunk/postgis/postgis.sql.in#L440

Reading through Postgres documentation I can't figure out what else shall
we do for this join to work. How do we make it work?

--
Darafei Praliaskouski
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