[GitHub] [spark] cloud-fan commented on a change in pull request #31595: [SPARK-34474][SQL] Remove unnecessary Union under Distinct/Deduplicate

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

cloud-fan commented on a change in pull request #31595:
URL: https://github.com/apache/spark/pull/31595#discussion_r581012033



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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@@ -501,6 +502,42 @@ object RemoveNoopOperators extends Rule[LogicalPlan] {
   }
 }
 
+/**
+ * Remove no-op `Union` from the query plan that do not make any modifications.
+ */
+object RemoveNoopUnion extends Rule[LogicalPlan] {
+  private def removeAliasOnlyProject(plan: LogicalPlan): LogicalPlan = plan match {
+    case p @ Project(projectList, child) =>
+      val originalOutputs = projectList.collect {

Review comment:
       In addition to adding tests, can we also add comments to explain it? `Project(a, a + 1 as res, Table(a))` is not alias-only but seems the code here will remove the `Project`.




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