viirya commented on a change in pull request #26509: [SPARK-29427][SQL] Add API 
to convert RelationalGroupedDataset to KeyValueGroupedDataset
URL: https://github.com/apache/spark/pull/26509#discussion_r346700405
 
 

 ##########
 File path: 
sql/core/src/main/scala/org/apache/spark/sql/RelationalGroupedDataset.scala
 ##########
 @@ -129,6 +132,32 @@ class RelationalGroupedDataset protected[sql](
     (inputExpr: Expression) => exprToFunc(inputExpr)
   }
 
+  /**
+   * Returns a `KeyValueGroupedDataset` where the data is grouped by the 
grouping expressions
+   * of current `RelationalGroupedDataset`.
+   *
+   * @since 3.0.0
+   */
+  def keyAs[U : Encoder]: KeyValueGroupedDataset[U, T] = {
+    val keyEncoder = encoderFor[U]
+    val aliasedGrps = groupingExprs.map { g =>
+     g.transformDown {
+       case u: UnresolvedAttribute => df.resolve(u.name)
+     }
+    }.map(alias)
+    val additionalCols = aliasedGrps.filter(g => 
!df.logicalPlan.outputSet.contains(g.toAttribute))
+    val qe = Dataset.ofRows(
+      df.sparkSession,
+      Project(df.logicalPlan.output ++ additionalCols, 
df.logicalPlan)).queryExecution
 
 Review comment:
   `KeyValueGroupedDataset` does not produce grouping attributes. These 
grouping attributes still come from the given `queryExecution`.
   
   If we change `groupingAttributes` to `Seq[NamedExpression]`, we still need 
add this `Project` to produce these grouping attributes.

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