viirya commented on a change in pull request #28560:
URL: https://github.com/apache/spark/pull/28560#discussion_r426391779



##########
File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/NestedColumnAliasing.scala
##########
@@ -68,10 +76,23 @@ object NestedColumnAliasing {
    */
   def replaceChildrenWithAliases(
       plan: LogicalPlan,
+      nestedFieldToAlias: Map[ExtractValue, Alias],
       attrToAliases: Map[ExprId, Seq[Alias]]): LogicalPlan = {
     plan.withNewChildren(plan.children.map { plan =>
       Project(plan.output.flatMap(a => attrToAliases.getOrElse(a.exprId, 
Seq(a))), plan)
-    })
+    }).transformExpressions {
+      case f: ExtractValue if nestedFieldToAlias.contains(f) =>
+        nestedFieldToAlias(f).toAttribute
+    }
+  }
+
+  /**
+   * Returns true for those operators that we can prune nested column on it.
+   */
+  private def canPruneOn(plan: LogicalPlan) = plan match {
+    case _: Aggregate => true
+    case _: Expand => true
+    case _ => false

Review comment:
       I think I was wrong. Re-checking `FlatMapGroupsInPandas`'s Python API, 
it looks like
   
   ```python
   df.groupby("id").apply(normalize).show()
   ```
   
   So basically the python udf takes no nested column selection but a full 
columns of DataFrame. It doesn't do nested column pruning.




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