Add UnaryCommand and make CreateTableAsSelect below it.

Author: viirya

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicOperators.scala

@@ -137,7 +137,7 @@ case class InsertIntoTable(
   }
 }
 
-trait CreateTableAsSelect extends UnaryNode {
+abstract class CreateTableAsSelect extends UnaryCommand {
   self: Product =>
   def databaseName: Option[String]
   def tableName: String

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/commands.scala

@@ -27,3 +27,11 @@ abstract class Command extends LeafNode {
   self: Product =>
   def output: Seq[Attribute] = Seq.empty
 }
+
+/**
+ * An UnaryNode that represents a non-query command to be executed by the system.
+ */
+abstract class UnaryCommand extends UnaryNode {
+  self: Product =>
+  def output: Seq[Attribute] = Seq.empty
+}