add omitted comment

Author: adrian-wang

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

@@ -90,6 +90,7 @@ private[sql] abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
            left.statistics.sizeInBytes <= sqlContext.conf.autoBroadcastJoinThreshold =>
           makeBroadcastHashJoin(leftKeys, rightKeys, left, right, condition, joins.BuildLeft)
 
+      // If the sort merge join option is set, we want to use sort merge join prior to hashjoin
       // for now let's support inner join first, then add outer join
       case ExtractEquiJoinKeys(Inner, leftKeys, rightKeys, condition, left, right)
         if sqlContext.conf.sortMergeJoinEnabled =>