[GitHub] [spark] peter-toth commented on a change in pull request #25479: [SPARK-28356][SHUFFLE][FOLLOWUP] Fix case with different pre-shuffle partition numbers

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peter-toth commented on a change in pull request #25479: [SPARK-28356][SHUFFLE][FOLLOWUP] Fix case with different pre-shuffle partition numbers
URL: https://github.com/apache/spark/pull/25479#discussion_r314814514
 
 

 ##########
 File path: sql/core/src/test/scala/org/apache/spark/sql/execution/ReduceNumShufflePartitionsSuite.scala
 ##########
 @@ -587,4 +587,22 @@ class ReduceNumShufflePartitionsSuite extends SparkFunSuite with BeforeAndAfterA
     }
     withSparkSession(test, 200, None)
   }
+
+  test("Union two datasets with different pre-shuffle partition number") {
+    val test: SparkSession => Unit = { spark: SparkSession =>
+      val df1 = spark.range(3).join(spark.range(3), "id").toDF()
+      val df2 = spark.range(3).groupBy().sum()
+
+      val resultDf = df1.union(df2)
+
+      checkAnswer(resultDf, Seq((0), (1), (2), (3)).map(i => Row(i)))
 
 Review comment:
   It does. The plan is:
   ```
   AdaptiveSparkPlan(isFinalPlan=false)
   +- Union
      :- Project [id#0L]
      :  +- SortMergeJoin [id#0L], [id#2L], Inner
      :     :- Sort [id#0L ASC NULLS FIRST], false, 0
      :     :  +- Exchange hashpartitioning(id#0L, 5), true
      :     :     +- Range (0, 3, step=1, splits=12)
      :     +- Sort [id#2L ASC NULLS FIRST], false, 0
      :        +- Exchange hashpartitioning(id#2L, 5), true
      :           +- Range (0, 3, step=1, splits=12)
      +- HashAggregate(keys=[], functions=[sum(id#6L)], output=[sum(id)#10L])
         +- Exchange SinglePartition, true
            +- HashAggregate(keys=[], functions=[partial_sum(id#6L)], output=[sum#14L])
               +- Range (0, 3, step=1, splits=12)
   ```
   and the error comes from this assert: https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/adaptive/ReduceNumShufflePartitions.scala#L136

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