Github user fhueske commented on a diff in the pull request: https://github.com/apache/flink/pull/1553#discussion_r53153386 --- Diff: flink-tests/src/test/java/org/apache/flink/test/javaApiOperators/GroupReduceITCase.java --- @@ -434,6 +431,95 @@ public void testCorrectnessOfGroupreduceWithDescendingGroupSort() throws Excepti } @Test + public void testCorrectnessOfGroupreduceWithExplicitPartitionOfReducer() throws Exception { + /* + * check correctness of groupReduce with descending group sort + */ + final ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment(); + env.setParallelism(1); --- End diff -- If we run this test with parallelism `1`, we cannot check if the partitioning is working correctly. Also use a custom `GroupReduceFunction` that implements the `GroupCombineFunction` interface. The combine function should be implemented in a way that we can actually verify that it was called.
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