spark RDD算子（六）之鍵值對聚合操做reduceByKey，foldByKey，排序操做sortByKey

reduceByKey def reduceByKey(func: (V, V) => V): RDD[(K, V)] def reduceByKey(func: (V, V) => V, numPartitions: Int): RDD[(K, V)] def reduceByKey(partitioner: Partitioner, func: (V, V) => V): RDD[(K,

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