Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/22957#discussion_r238626367
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/EnsureRequirements.scala
 ---
    @@ -145,9 +145,14 @@ case class EnsureRequirements(conf: SQLConf) extends 
Rule[SparkPlan] {
         assert(requiredChildDistributions.length == children.length)
         assert(requiredChildOrderings.length == children.length)
     
    +    val aliasMap = 
AttributeMap[Expression](children.flatMap(_.expressions.collect {
    +      case a: Alias => (a.toAttribute, a)
    +    }))
    +
         // Ensure that the operator's children satisfy their output 
distribution requirements.
         children = children.zip(requiredChildDistributions).map {
    -      case (child, distribution) if 
child.outputPartitioning.satisfies(distribution) =>
    +      case (child, distribution) if child.outputPartitioning.satisfies(
    +          distribution.mapExpressions(replaceAlias(_, aliasMap))) =>
    --- End diff --
    
    for example, `relation[a, b]`'s output partitioning is `[hash partition a, 
hash partition b]`, and `Project(a as c, b, relation)`'s output partitioning 
should be `[hash partition c, hash partition b]`.
    
    What do you mean by `But ProjectExec.outputPartitioning cannot contain a 
reference to the aliases`?


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