Hey all, Druid currently executes UNION ALL queries sequentially ( https://github.com/apache/incubator-druid/blob/master/sql/src/main/java/org/apache/druid/sql/calcite/rel/DruidUnionRel.java#L98). There's a comment in that method that restates this, but does not explain why. Is there a reason why each subquery of a union all query can't be executed in parallel?
Thanks, Julian
