[GitHub] [spark] cloud-fan commented on a change in pull request #24411: [SPARK-27514] Skip collapsing windows with empty window expressions

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cloud-fan commented on a change in pull request #24411: [SPARK-27514] Skip collapsing windows with empty window expressions
URL: https://github.com/apache/spark/pull/24411#discussion_r276879280
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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 @@ -770,6 +770,7 @@ object CollapseWindow extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
     case w1 @ Window(we1, ps1, os1, w2 @ Window(we2, ps2, os2, grandChild))
         if ps1 == ps2 && os1 == os2 && w1.references.intersect(w2.windowOutputSet).isEmpty &&
+          we1.nonEmpty && we2.nonEmpty &&
 
 Review comment:
   just for curiosity: do you know why we can't collapse windows with empty window expressions? Seems we can change `WindowFunctionType.functionType(we1.head) == WindowFunctionType.functionType(we2.head)` to `we1.isEmpty || we2.isEmpty || WindowFunctionType.functionType(we1.head) == WindowFunctionType.functionType(we2.head)` 

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