[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.

(cherry picked from commit a9ec033c8cf489898cc47e2043bd9e86b7df1ff8)
Signed-off-by: Michael Armbrust <michael@databricks.com>

2 files changed, 14 insertions, 1 deletions

diff --git a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
index fbab2ac16b..4b97dc25ac 100644
--- a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
+++ b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
@@ -218,6 +218,7 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
     val hiveContext = self
 
     override val strategies: Seq[Strategy] = Seq(
+      CommandStrategy(self),
       TakeOrdered,
       ParquetOperations,
       HiveTableScans,
@@ -304,7 +305,7 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
      */
     def stringResult(): Seq[String] = analyzed match {
       case NativeCommand(cmd) => runSqlHive(cmd)
-      case ExplainCommand(plan) => new QueryExecution { val logical = plan }.toString.split("\n")
+      case ExplainCommand(plan) => mkQueryExecution(plan).toString.split("\n")
       case query =>
         val result: Seq[Seq[Any]] = toRdd.collect().toSeq
         // We need the types so we can output struct field names
diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQuerySuite.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQuerySuite.scala
index 125cc18bfb..c56eee2580 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQuerySuite.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQuerySuite.scala
@@ -18,6 +18,7 @@
 package org.apache.spark.sql.hive.execution
 
 import org.apache.spark.sql.hive.test.TestHive._
+import org.apache.spark.sql.hive.test.TestHive
 
 /**
  * A set of test cases expressed in Hive QL that are not covered by the tests included in the hive distribution.
@@ -159,4 +160,15 @@ class HiveQuerySuite extends HiveComparisonTest {
     hql("SHOW TABLES").toString
     hql("SELECT * FROM src").toString
   }
+
+  test("SPARK-1704: Explain commands as a SchemaRDD") {
+    hql("CREATE TABLE IF NOT EXISTS src (key INT, value STRING)")
+    val rdd = hql("explain select key, count(value) from src group by key")
+    assert(rdd.collect().size == 1)
+    assert(rdd.toString.contains("ExplainCommand"))
+    assert(rdd.filter(row => row.toString.contains("ExplainCommand")).collect().size == 0,
+      "actual contents of the result should be the plans of the query to be explained")
+    TestHive.reset()
+  }
+
 }