Github user fhueske commented on the pull request:
https://github.com/apache/flink/pull/1734#issuecomment-190252285
Returning `false` does not imply that no plan can be found. It just means
that this branch of the plan space cannot be translated, but other rules such
as the rule for extracting expressions out of join conditions can lead to
branches that result in valid plans.
So we should not throw an exception in this rule (in fact in no rule).
However, we can catch the exception when Calcite did not find a plan and return
a more meaningful exception there.
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