[GitHub] [spark] c21 commented on a change in pull request #31595: [SPARK-34474][SQL] Remove unnecessary Union under Distinct/Deduplicate

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c21 commented on a change in pull request #31595:
URL: https://github.com/apache/spark/pull/31595#discussion_r580761460



##########
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:
       I feel `collect` is kind of dangerous here, we essentially ignore all expressions that are not attribute or alias of attribute here. How can it work correctly?
   
   I am thinking of example query like
   
   ```
   SELECT DISTINCT k, expr
   FROM
   (
     SELECT k, k + 1 AS expr
     FROM test_table
     UNION ALL
     SELECT k, k + 2 AS expr
     FROM test_table
   )
   ```




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