Show warning message if mapred.reduce.tasks is set to -1.

Author: viirya

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

@@ -84,8 +84,15 @@ case class SetCommand(
       logWarning(
         s"Property ${SQLConf.Deprecated.MAPRED_REDUCE_TASKS} is deprecated, " +
           s"automatically converted to ${SQLConf.SHUFFLE_PARTITIONS} instead.")
-      sqlContext.setConf(SQLConf.SHUFFLE_PARTITIONS, value)
-      Seq(Row(s"${SQLConf.SHUFFLE_PARTITIONS}=$value"))
+      if (value == "-1") {
+        logWarning(
+          s"Set this property to -1 for automatically determining the number of reducers " +
+            s"is not supported, showing current ${SQLConf.SHUFFLE_PARTITIONS} instead.")
+        Seq(Row(s"${SQLConf.SHUFFLE_PARTITIONS}=${sqlContext.conf.numShufflePartitions}"))
+      } else {
+        sqlContext.setConf(SQLConf.SHUFFLE_PARTITIONS, value)
+        Seq(Row(s"${SQLConf.SHUFFLE_PARTITIONS}=$value"))
+      }
 
     // Configures a single property.
     case Some((key, Some(value))) =>