[SPARK-2187] Explain should not run the optimizer twice.

@yhuai @marmbrus @concretevitamin

Author: Reynold Xin <rxin@apache.org>

Closes #1123 from rxin/explain and squashes the following commits:

def83b0 [Reynold Xin] Update unit tests for explain.
a9d3ba8 [Reynold Xin] [SPARK-2187] Explain should not run the optimizer twice.

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

@@ -250,9 +250,8 @@ private[sql] abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
     def apply(plan: LogicalPlan): Seq[SparkPlan] = plan match {
       case logical.SetCommand(key, value) =>
         Seq(execution.SetCommand(key, value, plan.output)(context))
-      case logical.ExplainCommand(child) =>
-        val sparkPlan = context.executePlan(child).sparkPlan
-        Seq(execution.ExplainCommand(sparkPlan, plan.output)(context))
+      case logical.ExplainCommand(logicalPlan) =>
+        Seq(execution.ExplainCommand(logicalPlan, plan.output)(context))
       case logical.CacheCommand(tableName, cache) =>
         Seq(execution.CacheCommand(tableName, cache)(context))
       case _ => Nil

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

@@ -21,6 +21,7 @@ import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.{SQLContext, Row}
 import org.apache.spark.sql.catalyst.expressions.{GenericRow, Attribute}
+import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 
 trait Command {
   /**
@@ -71,16 +72,23 @@ case class SetCommand(
 }
 
 /**
+ * An explain command for users to see how a command will be executed.
+ *
+ * Note that this command takes in a logical plan, runs the optimizer on the logical plan
+ * (but do NOT actually execute it).
+ *
  * :: DeveloperApi ::
  */
 @DeveloperApi
 case class ExplainCommand(
-    child: SparkPlan, output: Seq[Attribute])(
+    logicalPlan: LogicalPlan, output: Seq[Attribute])(
     @transient context: SQLContext)
-  extends UnaryNode with Command {
+  extends LeafNode with Command {
 
-  // Actually "EXPLAIN" command doesn't cause any side effect.
-  override protected[sql] lazy val sideEffectResult: Seq[String] = this.toString.split("\n")
+  // Run through the optimizer to generate the physical plan.
+  override protected[sql] lazy val sideEffectResult: Seq[String] = {
+    "Physical execution plan:" +: context.executePlan(logicalPlan).executedPlan.toString.split("\n")
+  }
 
   def execute(): RDD[Row] = {
     val explanation = sideEffectResult.map(row => new GenericRow(Array[Any](row)))

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

@@ -202,12 +202,9 @@ class HiveQuerySuite extends HiveComparisonTest {
     }
   }
 
-  private val explainCommandClassName =
-    classOf[execution.ExplainCommand].getSimpleName.stripSuffix("$")
-
   def isExplanation(result: SchemaRDD) = {
     val explanation = result.select('plan).collect().map { case Row(plan: String) => plan }
-    explanation.size > 1 && explanation.head.startsWith(explainCommandClassName)
+    explanation.size > 1 && explanation.head.startsWith("Physical execution plan")
   }
 
   test("SPARK-1704: Explain commands as a SchemaRDD") {