The two sides of the UNION are not equivalent because one of the files has an additional column, but Aman tells me that this should work in a schema-less environment anyway. (I was expecting it to fail but with a more meaningful error because I am used to relational database UNION queries where a corresponding number of columns is expected on either side of the UNION operator.)
The query fails whether or not the LIMIT is applied. EXPLAIN for this query also fails.
0: jdbc:drill:> select * from `clicks/clicks.campaign.json` union all select * from `clicks/clicks.json` limit 5;
Query failed: Failure while setting up Foreman. DrillUnionRel#23471 [159a4852-e190-4edf-b11e-2d62db8efbd7]
Error: exception while executing query: Failure while trying to get next result batch. (state=,code=0)