Github user cloud-fan commented on a diff in the pull request: https://github.com/apache/spark/pull/21072#discussion_r183392394 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala --- @@ -736,12 +736,29 @@ object EliminateSorts extends Rule[LogicalPlan] { } /** - * Removes Sort operation if the child is already sorted + * Removes redundant Sort operation. This can happen: + * 1) if the child is already sorted + * 2) if there is another Sort operator separated by 0...n Project/Filter operators */ object RemoveRedundantSorts extends Rule[LogicalPlan] { def apply(plan: LogicalPlan): LogicalPlan = plan transform { --- End diff -- assume the plan is ``` Sort Filter Sort Filter Sort OtherPlan ``` If we do `transformUp`, we hit the rule 3 times, which has some unnecessary transformation. If it's `transformDown`, it's one-shot.
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