[SPARK-2191][SQL] Make sure InsertIntoHiveTable doesn't execute more than once.

Author: Michael Armbrust <michael@databricks.com>

Closes #1129 from marmbrus/doubleCreateAs and squashes the following commits:

9c6d9e4 [Michael Armbrust] Fix typo.
5128fe2 [Michael Armbrust] Make sure InsertIntoHiveTable doesn't execute each time you ask for its result.

sql/hive/src/main/scala/org/apache/spark/sql/hive/execution/hiveOperators.scala

@@ -344,12 +344,16 @@ case class InsertIntoHiveTable(
     writer.commitJob()
   }
 
+  override def execute() = result
+
   /**
    * Inserts all the rows in the table into Hive.  Row objects are properly serialized with the
    * `org.apache.hadoop.hive.serde2.SerDe` and the
    * `org.apache.hadoop.mapred.OutputFormat` provided by the table definition.
+   *
+   * Note: this is run once and then kept to avoid double insertions.
    */
-  def execute() = {
+  private lazy val result: RDD[Row] = {
     val childRdd = child.execute()
     assert(childRdd != null)
 

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQuerySuite.scala

@@ -28,6 +28,12 @@ import org.apache.spark.sql.{SchemaRDD, execution, Row}
  */
 class HiveQuerySuite extends HiveComparisonTest {
 
+  test("CREATE TABLE AS runs once") {
+    hql("CREATE TABLE foo AS SELECT 1 FROM src LIMIT 1").collect()
+    assert(hql("SELECT COUNT(*) FROM foo").collect().head.getLong(0) === 1,
+      "Incorrect number of rows in created table")
+  }
+
   createQueryTest("between",
     "SELECT * FROM src WHERE key Between 1 and 2")