Some additional comments + small cleanup to remove an unused parameter

sql/core/src/main/scala/org/apache/spark/sql/execution/Exchange.scala

@@ -117,9 +117,8 @@ case class Exchange(newPartitioning: Partitioning, child: SparkPlan) extends Una
 
   @transient private lazy val sparkConf = child.sqlContext.sparkContext.getConf
 
-  private val rowDataTypes = child.output.map(_.dataType).toArray
-
   private val serializer: Serializer = {
+    val rowDataTypes = child.output.map(_.dataType).toArray
     // It is true when there is no field that needs to be write out.
     // For now, we will not use SparkSqlSerializer2 when noField is true.
     val noField = rowDataTypes == null || rowDataTypes.length == 0
@@ -181,7 +180,7 @@ case class Exchange(newPartitioning: Partitioning, child: SparkPlan) extends Una
         }
       }
     }
-    new ShuffledRowRDD(rowDataTypes, rddWithPartitionIds, serializer, part.numPartitions)
+    new ShuffledRowRDD(rddWithPartitionIds, serializer, part.numPartitions)
   }
 }
 

sql/core/src/main/scala/org/apache/spark/sql/execution/ShuffledRowRDD.scala

@@ -28,6 +28,10 @@ private class ShuffledRowRDDPartition(val idx: Int) extends Partition {
   override def hashCode(): Int = idx
 }
 
+/**
+ * A dummy partitioner for use with records whose partition ids have been pre-computed (i.e. for
+ * use on RDDs of (Int, Row) pairs where the Int is a partition id in the expected range).
+ */
 private class PartitionIdPassthrough(override val numPartitions: Int) extends Partitioner {
   override def getPartition(key: Any): Int = key.asInstanceOf[Int]
 }
@@ -37,9 +41,13 @@ private class PartitionIdPassthrough(override val numPartitions: Int) extends Pa
  * shuffling rows instead of Java key-value pairs. Note that something like this should eventually
  * be implemented in Spark core, but that is blocked by some more general refactorings to shuffle
  * interfaces / internals.
+ *
+ * @param prev the RDD being shuffled. Elements of this RDD are (partitionId, Row) pairs.
+ *             Partition ids should be in the range [0, numPartitions - 1].
+ * @param serializer the serializer used during the shuffle.
+ * @param numPartitions the number of post-shuffle partitions.
  */
 class ShuffledRowRDD(
-    rowSchema: Array[DataType],
     @transient var prev: RDD[Product2[Int, InternalRow]],
     serializer: Serializer,
     numPartitions: Int)