[SPARK-14350][SQL] EXPLAIN output should be in a single cell

## What changes were proposed in this pull request?

EXPLAIN output should be in a single cell.

**Before**
```
scala> sql("explain select 1").collect()
res0: Array[org.apache.spark.sql.Row] = Array([== Physical Plan ==], [WholeStageCodegen], [:  +- Project [1 AS 1#1]], [:     +- INPUT], [+- Scan OneRowRelation[]])
```

**After**
```
scala> sql("explain select 1").collect()
res1: Array[org.apache.spark.sql.Row] =
Array([== Physical Plan ==
WholeStageCodegen
:  +- Project [1 AS 1#4]
:     +- INPUT
+- Scan OneRowRelation[]])
```
Or,
```
scala> sql("explain select 1").head
res1: org.apache.spark.sql.Row =
[== Physical Plan ==
WholeStageCodegen
:  +- Project [1 AS 1#5]
:     +- INPUT
+- Scan OneRowRelation[]]
```

Please note that `Spark-shell(Scala-shell)` trims long string output. So, you may need to use `println` to get full strings.
```
scala> println(sql("explain codegen select 'a' as a group by 1").head)
[Found 2 WholeStageCodegen subtrees.
== Subtree 1 / 2 ==
WholeStageCodegen
...
/* 059 */   }
/* 060 */ }

]
```

## How was this patch tested?

Pass the Jenkins tests. (Testcases are updated.)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12137 from dongjoon-hyun/SPARK-14350.

2 files changed, 2 insertions, 2 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/command/commands.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/command/commands.scala
index 4bc62cdc4a..4eb8d7ff0d 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/command/commands.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/command/commands.scala
@@ -253,7 +253,7 @@ case class ExplainCommand(
       } else {
         queryExecution.simpleString
       }
-    outputString.split("\n").map(Row(_))
+    Seq(Row(outputString))
   } catch { case cause: TreeNodeException[_] =>
     ("Error occurred during query planning: \n" + cause.getMessage).split("\n").map(Row(_))
   }
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 58259060bf..5450368b88 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
@@ -707,7 +707,7 @@ class HiveQuerySuite extends HiveComparisonTest with BeforeAndAfter {
 
   def isExplanation(result: DataFrame): Boolean = {
     val explanation = result.select('plan).collect().map { case Row(plan: String) => plan }
-    explanation.contains("== Physical Plan ==")
+    explanation.head.startsWith("== Physical Plan ==")
   }
 
   test("SPARK-1704: Explain commands as a DataFrame") {