dataset的reparation和coalesce

  /**
   * Returns a new Dataset that has exactly `numPartitions` partitions, when the fewer partitions
   * are requested. If a larger number of partitions is requested, it will stay at the current
   * number of partitions. Similar to coalesce defined on an `RDD`, this operation results in
   * a narrow dependency, e.g. if you go from 1000 partitions to 100 partitions, there will not
   * be a shuffle, instead each of the 100 new partitions will claim 10 of the current partitions.
   *
   * However, if you're doing a drastic coalesce, e.g. to numPartitions = 1,
   * this may result in your computation taking place on fewer nodes than
   * you like (e.g. one node in the case of numPartitions = 1). To avoid this,
   * you can call repartition. This will add a shuffle step, but means the
   * current upstream partitions will be executed in parallel (per whatever
   * the current partitioning is).
   *
   * @group typedrel
   * @since 1.6.0
   */
  def coalesce(numPartitions: Int): Dataset[T] = withTypedPlan {
    Repartition(numPartitions, shuffle = false, planWithBarrier)
  }

关于coalsece：

1、用于减少分区数量，如果设置的numPartitions超过目前实际有的分区数，则分区数保持不变。

2、窄依赖，不会发生shuffle

3、极端的coalsece可能会影响性能，比如coalsece(1)，则只会在一个节点上运行单个任务。这种情况下建议使用repartition，

虽然repartition会发生shuffle，但是repartition对上游的计算，还是多分区并行执行的。

4、应用场景：多用于对一个大数据集filter以后，执行coalsece

  /**
   * Returns a new Dataset that has exactly `numPartitions` partitions.
   *
   * @group typedrel
   * @since 1.6.0
   */
  def repartition(numPartitions: Int): Dataset[T] = withTypedPlan {
    Repartition(numPartitions, shuffle = true, planWithBarrier)
  }

关于repartition：

跟coalsece一样，都是用于明确设置多少个分区，但是repartition是个宽依赖，会发生shuffle。主要注意的地方就是repartition不影响上游计算的分区

如果想极端的控制生成的文件数量来避免太多的小文件，建议repartition

测试：对一个大表查询，将查询的结果写到一张表

coalsece测试，coalsece(100)，可以看到只有一个stage，并且并行度是100

repartition测试，repartition(100)，发生了shuffle。两个stage，stage0对大表指定条件查询，对应的并行度默认是大表的数据量/128M，在repartition将结果输出到表的时候并行度为我们设置的repartition(100)，然后shuffle数据，最后输出

---

  /**
   * Return a new RDD that has exactly numPartitions partitions.
   *
   * Can increase or decrease the level of parallelism in this RDD. Internally, this uses
   * a shuffle to redistribute data.
   *
   * If you are decreasing the number of partitions in this RDD, consider using `coalesce`,
   * which can avoid performing a shuffle.
   *
   * TODO Fix the Shuffle+Repartition data loss issue described in SPARK-23207.
   */
  def repartition(numPartitions: Int)(implicit ord: Ordering[T] = null): RDD[T] = withScope {
    coalesce(numPartitions, shuffle = true)
  }

  /**
   * Return a new RDD that is reduced into `numPartitions` partitions.
   *
   * This results in a narrow dependency, e.g. if you go from 1000 partitions
   * to 100 partitions, there will not be a shuffle, instead each of the 100
   * new partitions will claim 10 of the current partitions. If a larger number
   * of partitions is requested, it will stay at the current number of partitions.
   *
   * However, if you're doing a drastic coalesce, e.g. to numPartitions = 1,
   * this may result in your computation taking place on fewer nodes than
   * you like (e.g. one node in the case of numPartitions = 1). To avoid this,
   * you can pass shuffle = true. This will add a shuffle step, but means the
   * current upstream partitions will be executed in parallel (per whatever
   * the current partitioning is).
   *
   * @note With shuffle = true, you can actually coalesce to a larger number
   * of partitions. This is useful if you have a small number of partitions,
   * say 100, potentially with a few partitions being abnormally large. Calling
   * coalesce(1000, shuffle = true) will result in 1000 partitions with the
   * data distributed using a hash partitioner. The optional partition coalescer
   * passed in must be serializable.
   */
  def coalesce(numPartitions: Int, shuffle: Boolean = false,
               partitionCoalescer: Option[PartitionCoalescer] = Option.empty)
              (implicit ord: Ordering[T] = null)
      : RDD[T] = withScope {
    require(numPartitions > 0, s"Number of partitions ($numPartitions) must be positive.")
    if (shuffle) {
      /** Distributes elements evenly across output partitions, starting from a random partition. */
      val distributePartition = (index: Int, items: Iterator[T]) => {
        var position = new Random(hashing.byteswap32(index)).nextInt(numPartitions)
        items.map { t =>
          // Note that the hash code of the key will just be the key itself. The HashPartitioner
          // will mod it with the number of total partitions.
          position = position + 1
          (position, t)
        }
      } : Iterator[(Int, T)]

      // include a shuffle step so that our upstream tasks are still distributed
      new CoalescedRDD(
        new ShuffledRDD[Int, T, T](mapPartitionsWithIndex(distributePartition),
        new HashPartitioner(numPartitions)),
        numPartitions,
        partitionCoalescer).values
    } else {
      new CoalescedRDD(this, numPartitions, partitionCoalescer)
    }
  }