关键字：Spark算子、Spark RDD基本转换、coalesce、repartition
coalesce
def coalesce(numPartitions: Int, shuffle: Boolean = false)(implicit ord: Ordering[T] = null): RDD[T]
该函数用于将RDD进行重分区，使用HashPartitioner。
第一个参数为重分区的数目，第二个为是否进行shuffle，默认为false;
以下面的例子来看：
scala> var data = sc.textFile("/tmp/lxw1234/1.txt")
data: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[53] at textFile at :21
scala> data.collect
res37: Array[String] = Array(hello world, hello spark, hello hive, hi spark)
scala> data.partitions.size
res38: Int = 2 //RDD data默认有两个分区
scala> var rdd1 = data.coalesce(1)
rdd1: org.apache.spark.rdd.RDD[String] = CoalescedRDD[2] at coalesce at :23
scala> rdd1.partitions.size
res1: Int = 1 //rdd1的分区数为1
scala> var rdd1 = data.coalesce(4)
rdd1: org.apache.spark.rdd.RDD[String] = CoalescedRDD[3] at coalesce at :23
scala> rdd1.partitions.size
res2: Int = 2 //如果重分区的数目大于原来的分区数，那么必须指定shuffle参数为true，//否则，分区数不便
scala> var rdd1 = data.coalesce(4,true)
rdd1: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[7] at coalesce at :23
scala> rdd1.partitions.size
res3: Int = 4
repartition
def repartition(numPartitions: Int)(implicit ord: Ordering[T] = null): RDD[T]
该函数其实就是coalesce函数第二个参数为true的实现
scala> var rdd2 = data.repartition(1) rdd2: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[11] at repartition at :23 scala> rdd2.partitions.size res4: Int = 1 scala> var rdd2 = data.repartition(4) rdd2: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[15] at repartition at :23 scala> rdd2.partitions.size res5: Int = 4
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