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MDEV-25080 Allow pushdown of queries involving UNIONs in outer select to foreign engines

To: Roman Nozdrin <roman.nozdrin@xxxxxxxxxxx>, Gagan Goel <gagan.goel@xxxxxxxxxxx>
From: Oleg Smirnov <oleg.smirnov@xxxxxxxxxxx>
Date: Fri, 9 Sep 2022 18:28:18 +0700
Cc: maria-developers@xxxxxxxxxxxxxxxxxxx, Sergei Golubchik <serg@xxxxxxxxxxx>

Hello!

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;

-- 
Best regards,
-------------------------
*Oleg Smirnov*
Sr. Software Engineer
MariaDB Server Team