Spark 重分区函数：coalesce和repartition区别与实现，能够优化Spark程序性能

源码包路径： org.apache.spark.rdd.RDDnode coalesce函数：redis 方法注释：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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