[GitHub] [spark] HeartSaVioR commented on a change in pull request #28830: [SPARK-31990][SQL] Preserves the input order of colNames in dropDuplicates

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

HeartSaVioR commented on a change in pull request #28830:
URL: https://github.com/apache/spark/pull/28830#discussion_r439903851



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File path: sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala
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@@ -2548,6 +2548,21 @@ class DataFrameSuite extends QueryTest
       assert(df.schema === new StructType().add(StructField("d", DecimalType(38, 0))))
     }
   }
+
+  test("SPARK-31990: preserves the input order of colNames in dropDuplicates") {
+    val df = Seq((1, 2, 3, 4, 5), (1, 2, 3, 4, 5)).toDF("c", "e", "d", "a", "b")
+    val inputColNames = Seq("c", "b", "c", "d", "b", "c", "b")

Review comment:
       You may want to find the case where `toSet.toSeq` produces different order than `distinct`.
   
   And then you may need to craft a streaming query doing dropDuplicate with having grouping key just you found, and run the query with Spark 2.4.x (where SPARK-31292 is not applied), and store checkpoint, and run the query with Spark 3.x (where SPARK-31292 is applied) with restoring checkpoint.
   
   There're a couple of test suites doing that - one of example is https://github.com/apache/spark/blob/8282bbf12d4e174986a649023ce3984aae7d7755/sql/core/src/test/scala/org/apache/spark/sql/streaming/FlatMapGroupsWithStateSuite.scala#L880-L952




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