[GitHub] [spark] dongjoon-hyun commented on a change in pull request #29053: [SPARK-32241][SQL] Remove empty children of union

[GitBox <gi...@apache.org>]

dongjoon-hyun commented on a change in pull request #29053:
URL: https://github.com/apache/spark/pull/29053#discussion_r452386261



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PropagateEmptyRelation.scala
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@@ -50,8 +50,25 @@ object PropagateEmptyRelation extends Rule[LogicalPlan] with PredicateHelper wit
   override def conf: SQLConf = SQLConf.get
 
   def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
-    case p: Union if p.children.forall(isEmptyLocalRelation) =>
-      empty(p)
+    case p @ Union(children) if children.exists(isEmptyLocalRelation) =>
+      val newChildren = children.filterNot(isEmptyLocalRelation)
+      if (newChildren.isEmpty) {
+        empty(p)
+      } else {

Review comment:
       Could you add a comment since this `else` body is non-trivial?




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