Spark 重分區函數：coalesce和repartition區別與實現，能夠優化Spark程序性能

源碼包路徑： org.apache.spark.rdd.RDDnode coalesce函數：redis 方法註釋：apache Return a new RDDthat is reduced into numPartitions partitions. This results in a narrowdependency, e.g. if you go from 1000 partitions

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