I noticed that in 10.3.x series there is apparently new JOIN push-down for engines to use.
See MDEV-7698. Hopefully his can be used for CONNECT JDBC.

It would be massively helpful if I had 3 CONNECT JDBC tables and I issued a SELECT joining those three tables with a WHERE clause on only one of the tables (a very common use case for us) if it could push down the JOIN in addition to the WHERE clause... at least in special cases where all the tables are CONNECT.

Right now, if each table has 500,000 rows and the select only returns 2,000 rows, it must pull back 3 x 500,000 rows to do the work... JOIN clause push down would offload this work to the remote server and only return the 2,000 row result.