[SPARK-19366][SQL] add getNumPartitions to Dataset

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

@@ -2420,6 +2420,13 @@ class Dataset[T] private[sql](
     RepartitionByExpression(partitionExprs.map(_.expr), logicalPlan, numPartitions = None)
   }
 
+  /**
+   * Returns the number of partitions of this Dataset.
+   * @group basic
+   * @since 2.2.0
+   */
+  def getNumPartitions: Int = rdd.getNumPartitions
+
   /**
    * Returns a new Dataset that has exactly `numPartitions` partitions.
    * Similar to coalesce defined on an `RDD`, this operation results in a narrow dependency, e.g.

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

@@ -269,6 +269,7 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
     checkAnswer(
       testData.select('key).repartition(10).select('key),
       testData.select('key).collect().toSeq)
+    assert(testData.select('key).repartition(10).select('key).getNumPartitions == 10)
   }
 
   test("coalesce") {
@@ -1301,12 +1302,14 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
   test("distributeBy and localSort") {
     val original = testData.repartition(1)
     assert(original.rdd.partitions.length == 1)
+    assert(original.getNumPartitions == 1)
     val df = original.repartition(5, $"key")
     assert(df.rdd.partitions.length == 5)
+    assert(df.getNumPartitions == 5)
     checkAnswer(original.select(), df.select())
 
     val df2 = original.repartition(10, $"key")
-    assert(df2.rdd.partitions.length == 10)
+    assert(df2.getNumPartitions == 10)
     checkAnswer(original.select(), df2.select())
 
     // Group by the column we are distributed by. This should generate a plan with no exchange