Github user fhueske commented on the pull request:
https://github.com/apache/flink/pull/1138#issuecomment-142723198
Hi @jkovacs, thanks for all your efforts to make the projection work. Going
for a `GenericeTypeInfo` would work in many cases but unfortunately not in all.
For example `union` operates in Flink on serialization level and requires that
all data sets which are unioned use the same serializer. By transparently using
a `GenericTypeInfo` users might be surprised why
`DataSet<Tuple2<String,Long>.union(DataSet<Tuple2<String,Long>)` does not work.
If we only support OuterJoins with an explicit JoinFunction, the user has full
control how to deal with null values and can even use a custom Tuple type or
Tuple serializer (via `Operator.returns()`) that supports null values. In my
opinion, the best approach is to only support OuterJoins with JoinFunctions.
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