Till Rohrmann created FLINK-2883:
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Summary: Combinable reduce produces wrong result
Key: FLINK-2883
URL: https://issues.apache.org/jira/browse/FLINK-2883
Project: Flink
Issue Type: Bug
Affects Versions: 0.10
Reporter: Till Rohrmann
If one uses a combinable reduce operation which also changes the key value of
the underlying data element, then the results of the reduce operation can
become wrong. The reason is that after the combine phase, another reduce
operator is executed which will then reduce the elements based on the new key
values. This might be not so surprising if one explicitly defined ones
{{GroupReduceOperation}} as combinable. However, the {{ReduceFunction}}
conceals the fact that a combiner is used implicitly. Furthermore, the API does
not prevent the user from changing the key fields which could solve the problem.
The following example program illustrates the problem
{code}
val env = ExecutionEnvironment.getExecutionEnvironment
env.setParallelism(1)
val input = env.fromElements((1,2), (1,3), (2,3), (3,3), (3,4))
val result = input.groupBy(0).reduce{
(left, right) =>
(left._1 + right._1, left._2 + right._2)
}
result.output(new PrintingOutputFormat[Int]())
env.execute()
{code}
The expected output is
{code}
(2, 5)
(2, 3)
(6, 7)
{code}
However, the actual output is
{code}
(4, 8)
(6, 7)
{code}
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