[jira] [Created] (SPARK-19981) Sort-Merge join inserts shuffles when joining dataframes with aliased columns

["Allen George (JIRA)" <ji...@apache.org>]

Allen George created SPARK-19981:
------------------------------------

             Summary: Sort-Merge join inserts shuffles when joining dataframes with aliased columns
                 Key: SPARK-19981
                 URL: https://issues.apache.org/jira/browse/SPARK-19981
             Project: Spark
          Issue Type: Bug
          Components: SQL
    Affects Versions: 2.0.2
            Reporter: Allen George


Performing a sort-merge join with two dataframes - each of which has the join column aliased - causes Spark to insert an unnecessary shuffle.

Consider the scala test code below, which should be equivalent to the following SQL.

{code:SQL}
SELECT * FROM
  (SELECT number AS aliased from df1) t1
LEFT JOIN
  (SELECT number AS aliased from df2) t2
ON t1.aliased = t2.aliased
{code}

{code:scala}
private case class OneItem(number: Long)
private case class TwoItem(number: Long, value: String)
test("join with aliases should not trigger shuffle") {
  val df1 = sqlContext.createDataFrame(
    Seq(
      OneItem(0),
      OneItem(2),
      OneItem(4)
    )
  )
  val partitionedDf1 = df1.repartition(10, col("number"))
  partitionedDf1.createOrReplaceTempView("df1")
  partitionedDf1.cache() partitionedDf1.count()
  
  val df2 = sqlContext.createDataFrame(
    Seq(
      TwoItem(0, "zero"),
      TwoItem(2, "two"),
      TwoItem(4, "four")
    )
  )
  val partitionedDf2 = df2.repartition(10, col("number"))
  partitionedDf2.createOrReplaceTempView("df2")
  partitionedDf2.cache() partitionedDf2.count()
  
  val fromDf1 = sqlContext.sql("SELECT number from df1")
  val fromDf2 = sqlContext.sql("SELECT number from df2")

  val aliasedDf1 = fromDf1.select(col(fromDf1.columns.head) as "aliased")
  val aliasedDf2 = fromDf2.select(col(fromDf2.columns.head) as "aliased")

  aliasedDf1.join(aliasedDf2, Seq("aliased"), "left_outer") }
{code}

Both the SQL and the Scala code generate a query-plan where an extra exchange is inserted before performing the sort-merge join. This exchange changes the partitioning from {{HashPartitioning("number", 10)}} for each frame being joined into {{HashPartitioning("aliased", 5)}}. I would have expected that since it's a simple column aliasing, and both frames have exactly the same partitioning that the initial frames.

{noformat} 
*Project [args=[aliased#267L]][outPart=PartitioningCollection(5, hashpartitioning(aliased#267L, 5)%NONNULL,hashpartitioning(aliased#270L, 5)%NONNULL)][outOrder=List(aliased#267L ASC%NONNULL)][output=List(aliased#267:bigint%NONNULL)]
+- *SortMergeJoin [args=[aliased#267L], [aliased#270L], Inner][outPart=PartitioningCollection(5, hashpartitioning(aliased#267L, 5)%NONNULL,hashpartitioning(aliased#270L, 5)%NONNULL)][outOrder=List(aliased#267L ASC%NONNULL)][output=ArrayBuffer(aliased#267:bigint%NONNULL, aliased#270:bigint%NONNULL)]
   :- *Sort [args=[aliased#267L ASC], false, 0][outPart=HashPartitioning(5, aliased#267:bigint%NONNULL)][outOrder=List(aliased#267L ASC%NONNULL)][output=ArrayBuffer(aliased#267:bigint%NONNULL)]
   :  +- Exchange [args=hashpartitioning(aliased#267L, 5)%NONNULL][outPart=HashPartitioning(5, aliased#267:bigint%NONNULL)][outOrder=List()][output=ArrayBuffer(aliased#267:bigint%NONNULL)]
   :     +- *Project [args=[number#198L AS aliased#267L]][outPart=HashPartitioning(10, number#198:bigint%NONNULL)][outOrder=List()][output=ArrayBuffer(aliased#267:bigint%NONNULL)]
   :        +- InMemoryTableScan [args=[number#198L]][outPart=HashPartitioning(10, number#198:bigint%NONNULL)][outOrder=List()][output=ArrayBuffer(number#198:bigint%NONNULL)]
   :           :  +- InMemoryRelation [number#198L], true, 10000, StorageLevel(disk, memory, deserialized, 1 replicas), false[Statistics(24,false)][output=List(number#198:bigint%NONNULL)]
   :           :     :  +- Exchange [args=hashpartitioning(number#198L, 10)%NONNULL][outPart=HashPartitioning(10, number#198:bigint%NONNULL)][outOrder=List()][output=List(number#198:bigint%NONNULL)]
   :           :     :     +- LocalTableScan [args=[number#198L]][outPart=UnknownPartitioning(0)][outOrder=List()][output=List(number#198:bigint%NONNULL)]
   +- *Sort [args=[aliased#270L ASC], false, 0][outPart=HashPartitioning(5, aliased#270:bigint%NONNULL)][outOrder=List(aliased#270L ASC%NONNULL)][output=ArrayBuffer(aliased#270:bigint%NONNULL)]
      +- Exchange [args=hashpartitioning(aliased#270L, 5)%NONNULL][outPart=HashPartitioning(5, aliased#270:bigint%NONNULL)][outOrder=List()][output=ArrayBuffer(aliased#270:bigint%NONNULL)]
         +- *Project [args=[number#223L AS aliased#270L]][outPart=HashPartitioning(10, number#223:bigint%NONNULL)][outOrder=List()][output=ArrayBuffer(aliased#270:bigint%NONNULL)]
            +- InMemoryTableScan [args=[number#223L]][outPart=HashPartitioning(10, number#223:bigint%NONNULL)][outOrder=List()][output=ArrayBuffer(number#223:bigint%NONNULL)]
               :  +- InMemoryRelation [number#223L, value#224], true, 10000, StorageLevel(disk, memory, deserialized, 1 replicas), false[Statistics(47,false)][output=List(number#223:bigint%NONNULL, value#224:string%NULL)]
               :     :  +- Exchange [args=hashpartitioning(number#223L, 10)%NONNULL][outPart=HashPartitioning(10, number#223:bigint%NONNULL)][outOrder=List()][output=List(number#223:bigint%NONNULL, value#224:string%NULL)]
               :     :     +- LocalTableScan [args=[number#223L, value#224]][outPart=UnknownPartitioning(0)][outOrder=List()][output=List(number#223:bigint%NONNULL, value#224:string%NULL)]
{noformat}



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