apache/incubator-flink — pull request #109 by fhueske (2014-09-02 14:00:56+00:00)
Title: [FLINK-1073] Enables sorted group input for GroupReduce combiners

`GroupReduceCombineDriver` and `SynchronousChainedCombineDriver` use separate comparators for sorting and grouping.
Enabling multiple comparators for a single input of a driver required some refactoring.