The implementation for MDEV-25080 is ready and pushed to the preview-10.11-mdev-25080-union-pushdown branch. The FederatedX storage engine is adjusted accordingly.

The example of how it works can be found at mysql-test/federated/federatedx_create_handlers.test:

--echo # Pushdown of the whole UNION
SELECT * from federated.t1 UNION SELECT * from federated.t2;
EXPLAIN SELECT * from federated.t1 UNION SELECT * from federated.t2;

--echo # Pushdown of a part of the UNION
SELECT * from federated.t1 UNION SELECT * from t3;
EXPLAIN SELECT * from federated.t1 UNION SELECT * from t3;
--