[SPARK-2177][SQL] describe table result contains only one column

```
scala> hql("describe src").collect().foreach(println)

[key                 	string              	None                ]
[value               	string              	None                ]
```

The result should contain 3 columns instead of one. This screws up JDBC or even the downstream consumer of the Scala/Java/Python APIs.

I am providing a workaround. We handle a subset of describe commands in Spark SQL, which are defined by ...
```
DESCRIBE [EXTENDED] [db_name.]table_name
```
All other cases are treated as Hive native commands.

Also, if we upgrade Hive to 0.13, we need to check the results of context.sessionState.isHiveServerQuery() to determine how to split the result. This method is introduced by https://issues.apache.org/jira/browse/HIVE-4545. We may want to set Hive to use JsonMetaDataFormatter for the output of a DDL statement (`set hive.ddl.output.format=json` introduced by https://issues.apache.org/jira/browse/HIVE-2822).

The link to JIRA: https://issues.apache.org/jira/browse/SPARK-2177

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1118 from yhuai/SPARK-2177 and squashes the following commits:

fd2534c [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
b9b9aa5 [Yin Huai] rxin's comments.
e7c4e72 [Yin Huai] Fix unit test.
656b068 [Yin Huai] 100 characters.
6387217 [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
8003cf3 [Yin Huai] Generate strings with the format like Hive for unit tests.
9787fff [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
440c5af [Yin Huai] rxin's comments.
f1a417e [Yin Huai] Update doc.
83adb2f [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
366f891 [Yin Huai] Add describe command.
74bd1d4 [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
342fdf7 [Yin Huai] Split to up to 3 parts.
725e88c [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2177
bb8bbef [Yin Huai] Split every string in the result of a describe command.

1 files changed, 21 insertions, 0 deletions

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
index f5d0834a49..acb1b0f4dc 100644
--- 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
@@ -121,3 +121,24 @@ case class CacheCommand(tableName: String, doCache: Boolean)(@transient context:
 
   override def output: Seq[Attribute] = Seq.empty
 }
+
+/**
+ * :: DeveloperApi ::
+ */
+@DeveloperApi
+case class DescribeCommand(child: SparkPlan, output: Seq[Attribute])(
+    @transient context: SQLContext)
+  extends LeafNode with Command {
+
+  override protected[sql] lazy val sideEffectResult: Seq[(String, String, String)] = {
+    Seq(("# Registered as a temporary table", null, null)) ++
+      child.output.map(field => (field.name, field.dataType.toString, null))
+  }
+
+  override def execute(): RDD[Row] = {
+    val rows = sideEffectResult.map {
+      case (name, dataType, comment) => new GenericRow(Array[Any](name, dataType, comment))
+    }
+    context.sparkContext.parallelize(rows, 1)
+  }
+}