[SPARK-15097][SQL] make Dataset.sqlContext a stable identifier for imports

## What changes were proposed in this pull request?
Make Dataset.sqlContext a lazy val so that its a stable identifier and can be used for imports.
Now this works again:
import someDataset.sqlContext.implicits._

## How was this patch tested?
Add unit test to DatasetSuite that uses the import show above.

Author: Koert Kuipers <koert@tresata.com>

Closes #12877 from koertkuipers/feat-sqlcontext-stable-import.

2 files changed, 15 insertions, 1 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala b/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala
index 08be94e8d4..1bea72c471 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala
@@ -211,7 +211,8 @@ class Dataset[T] private[sql](
 
   private implicit def classTag = unresolvedTEncoder.clsTag
 
-  def sqlContext: SQLContext = sparkSession.wrapped
+  // sqlContext must be val because a stable identifier is expected when you import implicits
+  @transient lazy val sqlContext: SQLContext = sparkSession.wrapped
 
   protected[sql] def resolve(colName: String): NamedExpression = {
     queryExecution.analyzed.resolveQuoted(colName, sparkSession.sessionState.analyzer.resolver)
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala
index eee21acf75..68a12b0622 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala
@@ -653,6 +653,11 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
 
     dataset.join(actual, dataset("user") === actual("id")).collect()
   }
+
+  test("SPARK-15097: implicits on dataset's sqlContext can be imported") {
+    val dataset = Seq(1, 2, 3).toDS()
+    checkDataset(DatasetTransform.addOne(dataset), 2, 3, 4)
+  }
 }
 
 case class OtherTuple(_1: String, _2: Int)
@@ -713,3 +718,11 @@ class JavaData(val a: Int) extends Serializable {
 object JavaData {
   def apply(a: Int): JavaData = new JavaData(a)
 }
+
+/** Used to test importing dataset.sqlContext.implicits._ */
+object DatasetTransform {
+  def addOne(ds: Dataset[Int]): Dataset[Int] = {
+    import ds.sqlContext.implicits._
+    ds.map(_ + 1)
+  }
+}