Github user fhueske commented on the pull request:
https://github.com/apache/flink/pull/1138#issuecomment-143252740
As I said in my previous comments, I would prefer to support projection
joins initially.
Instead, it would be good, if we could ensure by API design that an outer
join is always completed with an `with(JoinFunction)` call. One way to do this
would be to return a special `OuterJoinOperatorSetsPredicate` object when
`.where()` is called. This `OuterJoinOperatorSetsPredicate` does not return a
`JoinOperator` when `equalTo()` is called but an unfinished outer join that
only allows to call `with()`. That way, the regular join API would remain
stable. Or do you have a better idea to model the API in the right way?
If we don't allow DefaultJoin and ProjectJoin for outer joins, we can also
revert the corresponding changes.
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