Github user fhueske commented on the pull request:
https://github.com/apache/flink/pull/1553#issuecomment-183014873
If a `Partitioner` is implemented such that is does not partition based on
the key attribute, it cannot be used for a Reduce or GroupReduce transformation
anyways. Also users should expect that a combiner is applied if a
`ReduceFunction` or a `GroupReduceFunction` that implements a combine interface
is used.
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