HeartSaVioR commented on a change in pull request #28830:
URL: https://github.com/apache/spark/pull/28830#discussion_r439905543



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File path: sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala
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@@ -2541,7 +2542,20 @@ class Dataset[T] private[sql](
   def dropDuplicates(colNames: Seq[String]): Dataset[T] = withTypedPlan {
     val resolver = sparkSession.sessionState.analyzer.resolver
     val allColumns = queryExecution.analyzed.output
-    val groupCols = colNames.distinct.flatMap { (colName: String) =>
+    // SPARK-31990: We must preserve the input order of `colNames` because of 
the compatibility
+    // issue (the Streaming's state store depends on the `groupCols` order).
+    val orderPreservingDistinctColNames = {
+      val nameSeen = mutable.Set[String]()

Review comment:
       Oh I didn't see @maropu 's comment while I'm commenting. ;) Thanks for 
explaining.




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