advancedxy commented on code in PR #100:
URL: 
https://github.com/apache/arrow-datafusion-comet/pull/100#discussion_r1505779006


##########
spark/src/main/scala/org/apache/comet/CometSparkSessionExtensions.scala:
##########
@@ -451,6 +468,23 @@ class CometSparkSessionExtensions
       }
     }
   }
+
+  // CometExec already wraps a `ColumnarToRowExec` for row-based operators. 
Therefore,
+  // `ColumnarToRowExec` is redundant and can be eliminated.
+  //
+  // It was added during ApplyColumnarRulesAndInsertTransitions' 
insertTransitions phase when Spark
+  // requests row-based output such as `collect` call. It's correct to add a 
redundant
+  // `ColumnarToRowExec` for `CometExec`. However, for certain operators such 
as
+  // `CometCollectLimitExec` which overrides `executeCollect`, the redundant 
`ColumnarToRowExec`
+  // makes the override ineffective. The purpose of this rule is to eliminate 
the redundant
+  // `ColumnarToRowExec` for such operators.
+  case class EliminateRedundantColumnarToRow(session: SparkSession) extends 
Rule[SparkPlan] {
+    override def apply(plan: SparkPlan): SparkPlan = {
+      plan.transform { case ColumnarToRowExec(child: CometCollectLimitExec) =>
+        child

Review Comment:
   Fixed.



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