Spark sortMergeJoin 不会更改为 shuffleHashJoin

Question

我试图通过禁用 BroadcastHashJoin 和 SortMergeJoin 来强制 Spark 使用 ShuffleHashJoin，但 Spark 始终使用 SortMergeJoin。

我使用的是spark版本2.4.3

object ShuffleHashJoin {

def main(args: Array[String]): Unit = {
Logger.getLogger("org").setLevel(Level.ERROR)

val spark = SparkSession.builder()
  .appName("ShuffleHashJoin")
  .master("local[*]")
  .getOrCreate()

/*
* Disable auto broadcasting of table and SortMergeJoin
*/
spark.conf.set("spark.sql.autoBroadcastJoinThreshold", 0)
spark.conf.set("spark.sql.join.preferSortMergeJoin", false)

import spark.implicits._
val dataset = Seq(
  (0, "playing"),
  (1, "with"),
  (2, "ShuffledHashJoinExec")).toDF("id", "token")

dataset.join(dataset, Seq("id"), "inner").foreach(_ => ())

// infinite loop to keep the program running to check Spark UI at 4040 port.
while (true) {}

Answer 1

除了设置 Sparkspark.sql.join.preferSortMergeJoin之外false，还必须验证以下内容：（源代码）

1. 单个分区应该足够小以构建哈希表

canBuildLocalHashMap(right || left)
  |-> plan.stats.sizeInBytes < conf.autoBroadcastJoinThreshold * conf.numShufflePartitions

您已以编程方式设置spark.sql.autoBroadcastJoinThreshold为 0，因此它的计算结果始终为false。

2. 连接的一侧比另一侧小得多

   构建哈希映射的成本比排序更高，我们应该只在比其他表小得多的表上构建哈希映射。由于我们没有行数的统计信息，因此使用此处的字节大小作为估计。

muchSmaller(right, left) || muchSmaller(left, right) 
 |-> a.stats.sizeInBytes * 3 <= b.stats.sizeInBytes

在您的示例中，我们需要做几件事才能使其正常工作：

1. 将自动广播阈值更改为某个小值 spark.conf.set("spark.sql.autoBroadcastJoinThreshold", 2)

2. 至少使连接的一侧x3更大

以及工作示例：

    spark.conf.set("spark.sql.autoBroadcastJoinThreshold", 2)
    spark.conf.set("spark.sql.join.preferSortMergeJoin", false)

    import spark.implicits._
    val dataset = Seq(
      (0, "playing"),
      (1, "with"),
      (2, "ShuffledHashJoinExec")).toDF("id", "token")

    val right = Seq(
      (0, "asdfghjklzxcvb"),
      (1, "asdfghjklzxcvb"),
      (2, "asdfghjklzxcvb"),
      (3, "asdfghjklzxcvb"),
      (4, "asdfghjklzxcvb"),
      (5, "asdfghjklzxcvb"),
      (6, "asdfghjklzxcvb"),
      (7, "asdfghjklzxcvb"),
      (8, "asdfghjklzxcvb"),
      (9, "asdfghjklzxcvb"),
    )
      .toDF("id", "token")

    val joined = dataset.join(right, Seq("id"), "inner")
    joined.explain(true)

*(1) Project [id#5, token#6, token#15]
+- ShuffledHashJoin [id#5], [id#14], Inner, BuildLeft
   :- Exchange hashpartitioning(id#5, 200)
   :  +- LocalTableScan [id#5, token#6]
   +- Exchange hashpartitioning(id#14, 200)
      +- LocalTableScan [id#14, token#15]