Github user fhueske commented on the pull request: https://github.com/apache/flink/pull/1138#issuecomment-141111654 @jkovacs and @r-pogalz, thank you very much for this PR and the detailed description! It's quite a bit of code so it will take some time to be reviewed. I hope to give feedback soon. Nonetheless, we can start a discussion about the handling of projection for outer joins. By changing the type information to `GenericTypeInfo<TupleX>` to support tuples with null values, a `DataSet<TupleX>` cannot be used (in a join, groupBy, reduce, ...) as before because the runtime will use completely different serializers and comparators. Therefore, I am more in favor of not supporting projection for outer joins.
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