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

    https://github.com/apache/spark/pull/10577#discussion_r49798849
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/planning/patterns.scala
 ---
    @@ -170,17 +173,28 @@ object ExtractFiltersAndInnerJoins extends 
PredicateHelper {
       }
     }
     
    +
     /**
      * A pattern that collects all adjacent unions and returns their children 
as a Seq.
      */
     object Unions {
       def unapply(plan: LogicalPlan): Option[Seq[LogicalPlan]] = plan match {
    -    case u: Union => Some(collectUnionChildren(u))
    +    case u: Union => Some(collectUnionChildren(mutable.Stack(u), 
Seq.empty[LogicalPlan]))
         case _ => None
       }
     
    -  private def collectUnionChildren(plan: LogicalPlan): Seq[LogicalPlan] = 
plan match {
    -    case Union(l, r) => collectUnionChildren(l) ++ collectUnionChildren(r)
    -    case other => other :: Nil
    +  @tailrec
    +  private def collectUnionChildren(
    +      plans: mutable.Stack[LogicalPlan],
    +      children: Seq[LogicalPlan]): Seq[LogicalPlan] = {
    +    if (plans.isEmpty) children
    +    else {
    +      plans.pop match {
    +        case Union(grandchildren) =>
    +          grandchildren.reverseMap(plans.push(_))
    --- End diff --
    
    instead of reverse here, why not just use `Queue`?


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