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：

1. scala> var data = sc.textFile("/tmp/lxw1234/1.txt")
2. data: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[53] at textFile at :21
3.  
4. scala> data.collect
5. res37: Array[String] = Array(hello world, hello spark, hello hive, hi spark)
6.  
7. scala> data.partitions.size
8. res38: Int = 2 //RDD data默认有两个分区
9.  
10. scala> var rdd1 = data.coalesce(1)
11. rdd1: org.apache.spark.rdd.RDD[String] = CoalescedRDD[2] at coalesce at :23
12.  
13. scala> rdd1.partitions.size
14. res1: Int = 1 //rdd1的分区数为1
15.  
16.  
17. scala> var rdd1 = data.coalesce(4)
18. rdd1: org.apache.spark.rdd.RDD[String] = CoalescedRDD[3] at coalesce at :23
19.  
20. scala> rdd1.partitions.size
21. 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
22.  
23. scala> var rdd1 = data.coalesce(4,true)
24. rdd1: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[7] at coalesce at :23
25.  
26. scala> rdd1.partitions.size
27. res3: Int = 4
28.  

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

1. scala> var rdd2 = data.repartition(1)
2. rdd2: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[11] at repartition at :23
3.  
4. scala> rdd2.partitions.size
5. res4: Int = 1
6.  
7. scala> var rdd2 = data.repartition(4)
8. rdd2: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[15] at repartition at :23
9.  
10. scala> rdd2.partitions.size
11. res5: Int = 4

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