viirya commented on a change in pull request #31595:
URL: https://github.com/apache/spark/pull/31595#discussion_r581258723



##########
File path: 
sql/core/src/test/scala/org/apache/spark/sql/DataFrameSetOperationsSuite.scala
##########
@@ -808,6 +813,53 @@ class DataFrameSetOperationsSuite extends QueryTest with 
SharedSparkSession {
     // scalastyle:on
     checkAnswer(union, row1 :: row2 :: Nil)
   }
+
+  test("SPARK-34474: Remove unnecessary Union under Distinct") {
+    Seq(RemoveNoopUnion.ruleName, "").map { ruleName =>
+      withSQLConf(SQLConf.OPTIMIZER_EXCLUDED_RULES.key -> ruleName) {
+        val distinctUnionDF1 = testData.union(testData).distinct()
+        checkAnswer(distinctUnionDF1, testData.distinct())
+
+
+        val distinctUnionDF2 = 
testData.union(testData).dropDuplicates(Seq("key"))
+        checkAnswer(distinctUnionDF2, testData.dropDuplicates(Seq("key")))
+
+        val distinctUnionDF3 = sql(
+          """
+            |select key, value from testData
+            |union
+            |select key, value from testData
+            |""".stripMargin)
+        checkAnswer(distinctUnionDF3, testData.distinct())
+
+        val distinctUnionDF4 = sql(
+          """
+            |select distinct key, expr
+            |from
+            |(
+            |  select key, key + 1 as expr
+            |  from testData
+            |  union all
+            |  select key, key + 2 as expr
+            |  from testData
+            |)

Review comment:
       Oh, thanks. I think it is clear now. Let me fix it.




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