Makes DataFrame.rdd a lazy val

Author: liancheng

sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala

@@ -941,10 +941,10 @@ class DataFrame private[sql](
   /////////////////////////////////////////////////////////////////////////////
 
   /**
-   * Returns the content of the [[DataFrame]] as an [[RDD]] of [[Row]]s.
+   * Represents the content of the [[DataFrame]] as an [[RDD]] of [[Row]]s.
    * @group rdd
    */
-  def rdd: RDD[Row] = {
+  lazy val rdd: RDD[Row] = {
     // use a local variable to make sure the map closure doesn't capture the whole DataFrame
     val schema = this.schema
     queryExecution.executedPlan.execute().map(ScalaReflection.convertRowToScala(_, schema))