Github user fhueske commented on the issue:
https://github.com/apache/flink/pull/3141
The difference between failing during validation and translation is the
code line that throws the exception. A check during validation causes an
exception on the line that is responsible for the error. A check during
translation throws an exception much later in the user program.
Regarding the check in the rule: IMO, each rule must ensure that is does
not translate the query into an invalid plan. It make sense though, to have an
additional safety check in the `DataSetJoin`. However, this check should be
done in the constructor and not in the `translateToPlan()` method (see
FLINK-5547) to fail fast if a rule tries to create an invalid plan.
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