Github user twalthr commented on the issue:
https://github.com/apache/flink/pull/2938
Thanks for fixing the bug. The collection execution mode is on purpose
because we have too many IT cases at the moment. We should only test the
subclasses of `FlinkRel` like `DataSetUnion`, `DataStreamAggregate` etc. with
IT cases. Once this is the case we can reenable the cluster mode.
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