Github user fhueske commented on the issue:
https://github.com/apache/flink/pull/3141
I agree with @lincoln-lil and @shaoxuan-wang, this fix should go into the
`1.1-release`, `1.2-release`, and `master` branches. We will later add support
for non-equality predicate on left/right outer joins to master and include it
for 1.3.0.
Regarding the check, I agree with @wuchong. We implement these checks in
`RelOptRule.matches()` to prevent that an invalid plan is constructed.
Otherwise it may happen that we end up with an invalid plan although, there
would be a valid (but more expensive) alternative. We have additional checks in
the DataSet operators to be on the safe side but these should never be
triggered. If this happens, something is wrong with the optimization phase. So
its kind of a safety net. Although, I don't think there are rules to generate
alternative plans to execute an outer join with non-equality predicate, I would
like to keep the pattern for consistency reasons.
Also the check in the validation phase of the Table API is useful, because
the validation happens immediately when a function on a `Table` is called. In
contrast, the check on `DataSetJoin` would happen when the translation is
triggered, i.e., when the program is optimized and translated into a DataSet
program. Throwing an exception early is helpful for users because they can much
easier identify an invalid statement.
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