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Which is better groupByKey or reduceByKey?

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There are two different ways to compute counts:

val words1 = Array("one", "two", "two", "three", "three", "three")

val wordPairsRDD = sc.parallelize(words1).map(word => (word, 1))

val wordCountsWithReduce = wordPairsRDD .reduceByKey(_ + _) .collect() 

val wordCountsWithGroup = wordPairsRDD .groupByKey() .map(t => (t._1, t._2.sum)) .collect()

reduceByKey will aggregate y key before rearranging, and on the opposite hand, groupByKey can rearrange all the value key pairs because the diagrams show.

On large size data, the difference is obvious.

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