Github user fhueske commented on the issue:
https://github.com/apache/flink/pull/4324
Hi @hongyuhong, thanks for your feedback!
I agree, passing good error messages to users is important. However, this
is very difficult in the optimizer because the optimizer is exploring different
paths. If we throw an exception in the optimizer, we would kill the exploration
and miss a valid plan. That's why I'm very hesitant to throw exceptions in
rules. However, if the optimizer does not find a valid execution plan, it quits
with an error message that tells the user that no valid plan could be found but
does not point to a particular problem (such as an invalid join condition).
A good solution for this is still needed. Maybe we can collect potential
problems during optimization and provide these when the translation fails.
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