Github user gatorsmile commented on a diff in the pull request:

    https://github.com/apache/spark/pull/17331#discussion_r106797457
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala
 ---
    @@ -80,7 +80,18 @@ case class SortMergeJoinExec(
       override def requiredChildDistribution: Seq[Distribution] =
         ClusteredDistribution(leftKeys) :: ClusteredDistribution(rightKeys) :: 
Nil
     
    -  override def outputOrdering: Seq[SortOrder] = requiredOrders(leftKeys)
    +  override def outputOrdering: Seq[SortOrder] = joinType match {
    +    case RightOuter =>
    +      // For right outer join, values of the left key will be filled with 
nulls if it can't
    +      // match the value of the right key, so `nullOrdering` of the left 
key can't be guaranteed.
    +      // We should output right key order here.
    --- End diff --
    
    This comment is misleading. The output ordering is mainly affected by how 
we implement SortMergeJoinExec. 


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