[SPARK-1958] Calling .collect() on a SchemaRDD should call executeCollect() on...

[SPARK-1958] Calling .collect() on a SchemaRDD should call executeCollect() on the underlying query plan.

In cases like `Limit` and `TakeOrdered`, `executeCollect()` makes optimizations that `execute().collect()` will not.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #939 from liancheng/spark-1958 and squashes the following commits:

bdc4a14 [Cheng Lian] Copy rows to present immutable data to users
8250976 [Cheng Lian] Added return type explicitly for public API
192a25c [Cheng Lian] [SPARK-1958] Calling .collect() on a SchemaRDD should call executeCollect() on the underlying query plan.

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

@@ -368,6 +368,12 @@ class SchemaRDD(
     new SchemaRDD(sqlContext, SparkLogicalPlan(ExistingRdd(logicalPlan.output, rdd)))
   }
 
+  // =======================================================================
+  // Overriden RDD actions
+  // =======================================================================
+
+  override def collect(): Array[Row] = queryExecution.executedPlan.executeCollect()
+
   // =======================================================================
   // Base RDD functions that do NOT change schema
   // =======================================================================

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

@@ -49,7 +49,7 @@ abstract class SparkPlan extends QueryPlan[SparkPlan] with Logging {
   /**
    * Runs this query returning the result as an array.
    */
-  def executeCollect(): Array[Row] = execute().collect()
+  def executeCollect(): Array[Row] = execute().map(_.copy()).collect()
 
   protected def buildRow(values: Seq[Any]): Row =
     new GenericRow(values.toArray)

sql/core/src/test/scala/org/apache/spark/sql/parquet/ParquetQuerySuite.scala

@@ -201,7 +201,7 @@ class ParquetQuerySuite extends QueryTest with FunSuite with BeforeAndAfterAll {
   }
 
   test("insert (appending) to same table via Scala API") {
-    sql("INSERT INTO testsource SELECT * FROM testsource").collect()
+    sql("INSERT INTO testsource SELECT * FROM testsource")
     val double_rdd = sql("SELECT * FROM testsource").collect()
     assert(double_rdd != null)
     assert(double_rdd.size === 30)