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

    https://github.com/apache/spark/pull/13147#discussion_r64972400
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/planning/QueryPlanner.scala
 ---
    @@ -47,17 +55,27 @@ abstract class QueryPlanner[PhysicalPlan <: 
TreeNode[PhysicalPlan]] {
       /** A list of execution strategies that can be used by the planner */
       def strategies: Seq[GenericStrategy[PhysicalPlan]]
     
    -  /**
    -   * Returns a placeholder for a physical plan that executes `plan`. This 
placeholder will be
    -   * filled in automatically by the QueryPlanner using the other execution 
strategies that are
    -   * available.
    -   */
    -  protected def planLater(plan: LogicalPlan): PhysicalPlan = 
this.plan(plan).next()
    -
       def plan(plan: LogicalPlan): Iterator[PhysicalPlan] = {
         // Obviously a lot to do here still...
    -    val iter = strategies.view.flatMap(_(plan)).toIterator
    +    val iter = strategies.view.flatMap(_(plan)).toIterator.flatMap { 
physicalPlan =>
    +      val placeholders = collectPlaceholders(physicalPlan)
    +
    +      (Iterator(physicalPlan) /: placeholders.toIterator) {
    +        case (physicalPlans, (placeholder, logicalPlan)) =>
    +          val children = this.plan(logicalPlan)
    +          physicalPlans.flatMap { physicalPlan =>
    +            children.map { child =>
    +              physicalPlan.transformUp {
    +                case `placeholder` => child
    --- End diff --
    
    You might also explicitly spell out what is happening here.  It's doing the 
following right?
    
    ```scala
    case p if p == placeholder => child
    ```


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