[SQL] Make dataframe more tolerant of being serialized

Eases use in the spark-shell.

Author: Michael Armbrust <michael@databricks.com>

Closes apache#4545 from marmbrus/serialization and squashes the following commits:

04748e6 [Michael Armbrust] @scala.annotation.varargs
b36e219 [Michael Armbrust] moreFixes

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

@@ -76,7 +76,7 @@ private[sql] object DataFrame {
  */
 // TODO: Improve documentation.
 @Experimental
-trait DataFrame extends RDDApi[Row] {
+trait DataFrame extends RDDApi[Row] with Serializable {
 
   val sqlContext: SQLContext
 

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

@@ -44,8 +44,8 @@ import org.apache.spark.sql.types.{NumericType, StructType}
  * Internal implementation of [[DataFrame]]. Users of the API should use [[DataFrame]] directly.
  */
 private[sql] class DataFrameImpl protected[sql](
-    override val sqlContext: SQLContext,
-    val queryExecution: SQLContext#QueryExecution)
+    @transient override val sqlContext: SQLContext,
+    @transient val queryExecution: SQLContext#QueryExecution)
   extends DataFrame {
 
   /**

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

@@ -166,6 +166,15 @@ object Dsl {
   //////////////////////////////////////////////////////////////////////////////////////////////
   //////////////////////////////////////////////////////////////////////////////////////////////
 
+  /**
+   * Returns the first column that is not null.
+   * {{{
+   *   df.select(coalesce(df("a"), df("b")))
+   * }}}
+   */
+  @scala.annotation.varargs
+  def coalesce(e: Column*): Column = Coalesce(e.map(_.expr))
+
   /**
    * Unary minus, i.e. negate the expression.
    * {{{

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

@@ -129,9 +129,11 @@ class SQLContext(@transient val sparkContext: SparkContext)
    * A collection of methods that are considered experimental, but can be used to hook into
    * the query planner for advanced functionalities.
    */
+  @transient
   val experimental: ExperimentalMethods = new ExperimentalMethods(this)
 
   /** Returns a [[DataFrame]] with no rows or columns. */
+  @transient
   lazy val emptyDataFrame = DataFrame(this, NoRelation)
 
   /**
@@ -178,7 +180,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
    * (Scala-specific)
    * Implicit methods available in Scala for converting common Scala objects into [[DataFrame]]s.
    */
-  object implicits {
+  object implicits extends Serializable {
     // scalastyle:on
 
     /** Creates a DataFrame from an RDD of case classes or tuples. */