Throw exception instead of warning message.

Author: viirya

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

@@ -85,10 +85,9 @@ case class SetCommand(
         s"Property ${SQLConf.Deprecated.MAPRED_REDUCE_TASKS} is deprecated, " +
           s"automatically converted to ${SQLConf.SHUFFLE_PARTITIONS} instead.")
       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}"))
+        val msg = s"Setting ${SQLConf.Deprecated.MAPRED_REDUCE_TASKS} to -1 for automatically " +
+          "determining the number of reducers is not supported."
+        throw new IllegalArgumentException(msg)
       } else {
         sqlContext.setConf(SQLConf.SHUFFLE_PARTITIONS, value)
         Seq(Row(s"${SQLConf.SHUFFLE_PARTITIONS}=$value"))

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -830,6 +830,13 @@ class SQLQuerySuite extends QueryTest with BeforeAndAfterAll {
     conf.clear()
   }
 
+  test("SET commands with illegal or inappropriate argument") {
+    conf.clear()
+    // mapred.reduce.tasks=-1 for automatically determing the number of reducers is not supported
+    intercept[IllegalArgumentException](sql(s"SET mapred.reduce.tasks=-1"))
+    conf.clear()
+  }
+
   test("apply schema") {
     val schema1 = StructType(
       StructField("f1", IntegerType, false) ::