Revise comments; add handler to guard against future ShuffleManager i…

…mplementations

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

@@ -21,6 +21,7 @@ import org.apache.spark.{HashPartitioner, Partitioner, RangePartitioner, SparkEn
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.rdd.{RDD, ShuffledRDD}
 import org.apache.spark.serializer.Serializer
+import org.apache.spark.shuffle.hash.HashShuffleManager
 import org.apache.spark.shuffle.sort.SortShuffleManager
 import org.apache.spark.shuffle.unsafe.UnsafeShuffleManager
 import org.apache.spark.sql.catalyst.errors.attachTree
@@ -92,8 +93,11 @@ case class Exchange(
       } else if (serializeMapOutputs && serializer.supportsRelocationOfSerializedObjects) {
         // SPARK-4550 extended sort-based shuffle to serialize individual records prior to sorting
         // them. This optimization is guarded by a feature-flag and is only applied in cases where
-        // shuffle dependency does not specify an aggregator and the record serializer has certain
-        // properties. If this optimization is enabled, we can safely avoid the copy.
+        // shuffle dependency does not specify an aggregator or ordering and the record serializer
+        // has certain properties. If this optimization is enabled, we can safely avoid the copy.
+        //
+        // Exchange never configures its ShuffledRDDs with aggregators or key orderings, so we only
+        // need to check whether the optimization is enabled and supported by our serializer.
         //
         // This optimization also applies to UnsafeShuffleManager (added in SPARK-7081).
         false
@@ -104,9 +108,12 @@ case class Exchange(
         // both cases, we must copy.
         true
       }
-    } else {
+    } else if (shuffleManager.isInstanceOf[HashShuffleManager]) {
       // We're using hash-based shuffle, so we don't need to copy.
       false
+    } else {
+      // Catch-all case to safely handle any future ShuffleManager implementations.
+      true
     }
   }