[SPARK-13884][SQL] Remove DescribeCommand's dependency on LogicalPlan

## What changes were proposed in this pull request?
This patch removes DescribeCommand's dependency on LogicalPlan. After this patch, DescribeCommand simply accepts a TableIdentifier. It minimizes the dependency, and blocks my next patch (removes SQLContext dependency from SparkPlanner).

## How was this patch tested?
Should be covered by existing unit tests and Hive compatibility tests that run describe table.

Author: Reynold Xin <rxin@databricks.com>

Closes #11710 from rxin/SPARK-13884.

4 files changed, 12 insertions, 12 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkQl.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkQl.scala
index 8dde308f96..11391bd12a 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkQl.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkQl.scala
@@ -271,15 +271,14 @@ private[sql] class SparkQl(conf: ParserConf = SimpleParserConf()) extends Cataly
                   // issue.
                   val tableIdent = TableIdentifier(
                     cleanIdentifier(tableName), Some(cleanIdentifier(dbName)))
-                  datasources.DescribeCommand(
-                    UnresolvedRelation(tableIdent, None), isExtended = extended.isDefined)
+                  datasources.DescribeCommand(tableIdent, isExtended = extended.isDefined)
                 case Token(dbName, Nil) :: Token(tableName, Nil) :: Token(colName, Nil) :: Nil =>
                   // It is describing a column with the format like "describe db.table column".
                   nodeToDescribeFallback(node)
                 case tableName :: Nil =>
                   // It is describing a table with the format like "describe table".
                   datasources.DescribeCommand(
-                    UnresolvedRelation(TableIdentifier(cleanIdentifier(tableName.text)), None),
+                    TableIdentifier(cleanIdentifier(tableName.text)),
                     isExtended = extended.isDefined)
                 case _ =>
                   nodeToDescribeFallback(node)
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 bae0750788..6352c48c76 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
@@ -398,11 +398,10 @@ private[sql] abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
         sys.error("Tables created with SQLContext must be TEMPORARY. Use a HiveContext instead.")
 
       case describe @ LogicalDescribeCommand(table, isExtended) =>
-        val resultPlan = self.sqlContext.executePlan(table).executedPlan
-        ExecutedCommand(
-          RunnableDescribeCommand(resultPlan, describe.output, isExtended)) :: Nil
+        ExecutedCommand(RunnableDescribeCommand(table, describe.output, isExtended)) :: Nil
 
-      case logical.ShowFunctions(db, pattern) => ExecutedCommand(ShowFunctions(db, pattern)) :: Nil
+      case logical.ShowFunctions(db, pattern) =>
+        ExecutedCommand(ShowFunctions(db, pattern)) :: Nil
 
       case logical.DescribeFunction(function, extended) =>
         ExecutedCommand(DescribeFunction(function, extended)) :: Nil
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 54cdcb10ac..6e36a15a6d 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
@@ -22,7 +22,7 @@ import java.util.NoSuchElementException
 import org.apache.spark.Logging
 import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.{Dataset, Row, SQLContext}
-import org.apache.spark.sql.catalyst.{CatalystTypeConverters, InternalRow}
+import org.apache.spark.sql.catalyst.{CatalystTypeConverters, InternalRow, TableIdentifier}
 import org.apache.spark.sql.catalyst.errors.TreeNodeException
 import org.apache.spark.sql.catalyst.expressions.{Attribute, AttributeReference}
 import org.apache.spark.sql.catalyst.plans.logical
@@ -293,13 +293,14 @@ case object ClearCacheCommand extends RunnableCommand {
 
 
 case class DescribeCommand(
-    child: SparkPlan,
+    table: TableIdentifier,
     override val output: Seq[Attribute],
     isExtended: Boolean)
   extends RunnableCommand {
 
   override def run(sqlContext: SQLContext): Seq[Row] = {
-    child.schema.fields.map { field =>
+    val relation = sqlContext.sessionState.catalog.lookupRelation(table)
+    relation.schema.fields.map { field =>
       val cmtKey = "comment"
       val comment = if (field.metadata.contains(cmtKey)) field.metadata.getString(cmtKey) else ""
       Row(field.name, field.dataType.simpleString, comment)
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/ddl.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/ddl.scala
index 903c9913ac..04e51735c4 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/ddl.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/ddl.scala
@@ -33,8 +33,9 @@ import org.apache.spark.sql.types._
  *                   It is effective only when the table is a Hive table.
  */
 case class DescribeCommand(
-    table: LogicalPlan,
-    isExtended: Boolean) extends LogicalPlan with logical.Command {
+    table: TableIdentifier,
+    isExtended: Boolean)
+  extends LogicalPlan with logical.Command {
 
   override def children: Seq[LogicalPlan] = Seq.empty