Next clause

SELECT f1 from Source as s ORDER BY s.id

isn't supported by Qserv, indeed merge table has an id, not s.id column.

Adding ORDER BY fields in SELECT clause using alias would solve this:

SELECT f1, s.Id as __QSERV_ORDER_BY_1 from Source as s ORDER BY __QSERV_ORDER_BY_1

Nevertheless SELECT * FROM resultTable ORDER BY __QSERV_ORDER_BY_1 query on mysql-proxy would have to be replaced with something like SELECT f1 FROM resultTable ORDER BY __QSERV_ORDER_BY_1 (this select would take all columns except the __QSERV_ORDER_BY ones).