[SPARK-1704][SQL] Fully support EXPLAIN commands as SchemaRDD.

This PR attempts to resolve [SPARK-1704](https://issues.apache.org/jira/browse/SPARK-1704) by introducing a physical plan for EXPLAIN commands, which just prints out the debug string (containing various SparkSQL's plans) of the corresponding QueryExecution for the actual query.

Author: Zongheng Yang <zongheng.y@gmail.com>

Closes #1003 from concretevitamin/explain-cmd and squashes the following commits:

5b7911f [Zongheng Yang] Add a regression test.
1bfa379 [Zongheng Yang] Modify output().
719ada9 [Zongheng Yang] Override otherCopyArgs for ExplainCommandPhysical.
4318fd7 [Zongheng Yang] Make all output one Row.
439c6ab [Zongheng Yang] Minor cleanups.
408f574 [Zongheng Yang] SPARK-1704: Add CommandStrategy and ExplainCommandPhysical.

3 files changed, 49 insertions, 0 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala b/sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala
index 5626f0da22..fde4c485b5 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala
@@ -191,6 +191,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
     val sparkContext = self.sparkContext
 
     val strategies: Seq[Strategy] =
+      CommandStrategy(self) ::
       TakeOrdered ::
       PartialAggregation ::
       LeftSemiJoin ::
@@ -256,6 +257,11 @@ class SQLContext(@transient val sparkContext: SparkContext)
       Batch("Prepare Expressions", Once, new BindReferences[SparkPlan]) :: Nil
   }
 
+  // TODO: or should we make QueryExecution protected[sql]?
+  protected[sql] def mkQueryExecution(plan: LogicalPlan) = new QueryExecution {
+    val logical = plan
+  }
+
   /**
    * The primary workflow for executing relational queries using Spark.  Designed to allow easy
    * access to the intermediate phases of query execution for developers.
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
index 6463f47510..295c265b16 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala
@@ -233,4 +233,15 @@ private[sql] abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
       case _ => Nil
     }
   }
+
+  // TODO: this should be merged with SPARK-1508's SetCommandStrategy
+  case class CommandStrategy(context: SQLContext) extends Strategy {
+    def apply(plan: LogicalPlan): Seq[SparkPlan] = plan match {
+      case logical.ExplainCommand(child) =>
+        val qe = context.mkQueryExecution(child)
+        Seq(execution.ExplainCommandPhysical(qe.executedPlan, plan.output)(context))
+      case _ => Nil
+    }
+  }
+
 }
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/commands.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/commands.scala
new file mode 100644
index 0000000000..5371d2f479
--- /dev/null
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/commands.scala
@@ -0,0 +1,32 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.execution
+
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.{SQLContext, Row}
+import org.apache.spark.sql.catalyst.expressions.{GenericRow, Attribute}
+
+case class ExplainCommandPhysical(child: SparkPlan, output: Seq[Attribute])
+                                 (@transient context: SQLContext) extends UnaryNode {
+  def execute(): RDD[Row] = {
+    val planString = new GenericRow(Array[Any](child.toString))
+    context.sparkContext.parallelize(Seq(planString))
+  }
+
+  override def otherCopyArgs = context :: Nil
+}