[GitHub] [spark] cloud-fan commented on a change in pull request #28326: [SPARK-27340][SS] Alias on TimeWindow expression cause watermark metadata lost

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cloud-fan commented on a change in pull request #28326:
URL: https://github.com/apache/spark/pull/28326#discussion_r415531970



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
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@@ -3262,10 +3262,21 @@ object CleanupAliases extends Rule[LogicalPlan] {
 
   def trimNonTopLevelAliases(e: Expression): Expression = e match {
     case a: Alias =>
-      a.copy(child = trimAliases(a.child))(
+      val newChild = trimAliases(a.child)
+      // Specific logic for keeping the eventTime watermark metadata in the top level alias.

Review comment:
       can we make it general? I think what we need to do is to propagate the alias metadata when there are contiguous Alias nodes and we merge them into one.




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