[SPARK-2918] [SQL] Support the CTAS in EXPLAIN command

Hive supports the `explain` the CTAS, which was supported by Spark SQL previously, however, seems it was reverted after the code refactoring in HiveQL.

Author: Cheng Hao <hao.cheng@intel.com>

Closes apache#3357 from chenghao-intel/explain and squashes the following commits:

7aace63 [Cheng Hao] Support the CTAS in EXPLAIN command

(cherry picked from commit 6aa0fc9)
Signed-off-by: Michael Armbrust <michael@databricks.com>

Author: chenghao-intel

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveQl.scala

@@ -124,7 +124,6 @@ private[hive] object HiveQl {
 
   // Commands that we do not need to explain.
   protected val noExplainCommands = Seq(
-    "TOK_CREATETABLE",
     "TOK_DESCTABLE",
     "TOK_TRUNCATETABLE"     // truncate table" is a NativeCommand, does not need to explain.
   ) ++ nativeCommands
@@ -421,6 +420,11 @@ private[hive] object HiveQl {
     case Token("TOK_EXPLAIN", explainArgs)
       if noExplainCommands.contains(explainArgs.head.getText) =>
       ExplainCommand(NoRelation)
+    case Token("TOK_EXPLAIN", explainArgs)
+      if "TOK_CREATETABLE" == explainArgs.head.getText =>
+      val Some(crtTbl) :: _ :: extended :: Nil =
+        getClauses(Seq("TOK_CREATETABLE", "FORMATTED", "EXTENDED"), explainArgs)
+      ExplainCommand(nodeToPlan(crtTbl), extended != None)
     case Token("TOK_EXPLAIN", explainArgs) =>
       // Ignore FORMATTED if present.
       val Some(query) :: _ :: extended :: Nil =

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

@@ -38,4 +38,40 @@ class HiveExplainSuite extends QueryTest {
                    "== Physical Plan ==",
                    "Code Generation", "== RDD ==")
   }
+
+  test("explain create table command") {
+    checkExistence(sql("explain create table temp__b as select * from src limit 2"), true,
+                   "== Physical Plan ==",
+                   "InsertIntoHiveTable",
+                   "Limit",
+                   "src")
+
+    checkExistence(sql("explain extended create table temp__b as select * from src limit 2"), true,
+      "== Parsed Logical Plan ==",
+      "== Analyzed Logical Plan ==",
+      "== Optimized Logical Plan ==",
+      "== Physical Plan ==",
+      "CreateTableAsSelect",
+      "InsertIntoHiveTable",
+      "Limit",
+      "src")
+
+    checkExistence(sql(
+      """
+        | EXPLAIN EXTENDED CREATE TABLE temp__b
+        | ROW FORMAT SERDE "org.apache.hadoop.hive.serde2.columnar.ColumnarSerDe"
+        | WITH SERDEPROPERTIES("serde_p1"="p1","serde_p2"="p2")
+        | STORED AS RCFile
+        | TBLPROPERTIES("tbl_p1"="p11", "tbl_p2"="p22")
+        | AS SELECT * FROM src LIMIT 2
+      """.stripMargin), true,
+      "== Parsed Logical Plan ==",
+      "== Analyzed Logical Plan ==",
+      "== Optimized Logical Plan ==",
+      "== Physical Plan ==",
+      "CreateTableAsSelect",
+      "InsertIntoHiveTable",
+      "Limit",
+      "src")
+  }
 }