[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.

2 files changed, 37 insertions, 34 deletions

diff --git a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveStrategies.scala b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveStrategies.scala
index a19dc21638..f44937ec6f 100644
--- a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveStrategies.scala
+++ b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveStrategies.scala
@@ -102,18 +102,8 @@ private[hive] trait HiveStrategies {
   case class HiveCommandStrategy(context: HiveContext) extends Strategy {
     def apply(plan: LogicalPlan): Seq[SparkPlan] = plan match {
       case describe: DescribeCommand =>
-        val resolvedTable = context.executePlan(describe.table).analyzed
-        resolvedTable match {
-          case t: MetastoreRelation =>
-            ExecutedCommand(
-              DescribeHiveTableCommand(t, describe.output, describe.isExtended)) :: Nil
-
-          case o: LogicalPlan =>
-            val resultPlan = context.executePlan(o).executedPlan
-            ExecutedCommand(RunnableDescribeCommand(
-              resultPlan, describe.output, describe.isExtended)) :: Nil
-        }
-
+        ExecutedCommand(
+          DescribeHiveTableCommand(describe.table, describe.output, describe.isExtended)) :: Nil
       case _ => Nil
     }
   }
diff --git a/sql/hive/src/main/scala/org/apache/spark/sql/hive/execution/DescribeHiveTableCommand.scala b/sql/hive/src/main/scala/org/apache/spark/sql/hive/execution/DescribeHiveTableCommand.scala
index 57293fce97..8481324086 100644
--- a/sql/hive/src/main/scala/org/apache/spark/sql/hive/execution/DescribeHiveTableCommand.scala
+++ b/sql/hive/src/main/scala/org/apache/spark/sql/hive/execution/DescribeHiveTableCommand.scala
@@ -22,8 +22,10 @@ import scala.collection.JavaConverters._
 import org.apache.hadoop.hive.metastore.api.FieldSchema
 
 import org.apache.spark.sql.{Row, SQLContext}
+import org.apache.spark.sql.catalyst.TableIdentifier
 import org.apache.spark.sql.catalyst.expressions.Attribute
-import org.apache.spark.sql.execution.command.RunnableCommand
+import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
+import org.apache.spark.sql.execution.command.{DescribeCommand, RunnableCommand}
 import org.apache.spark.sql.hive.MetastoreRelation
 
 /**
@@ -31,33 +33,44 @@ import org.apache.spark.sql.hive.MetastoreRelation
  */
 private[hive]
 case class DescribeHiveTableCommand(
-    table: MetastoreRelation,
+    tableId: TableIdentifier,
     override val output: Seq[Attribute],
     isExtended: Boolean) extends RunnableCommand {
 
   override def run(sqlContext: SQLContext): Seq[Row] = {
-    // Trying to mimic the format of Hive's output. But not exactly the same.
-    var results: Seq[(String, String, String)] = Nil
-
-    val columns: Seq[FieldSchema] = table.hiveQlTable.getCols.asScala
-    val partitionColumns: Seq[FieldSchema] = table.hiveQlTable.getPartCols.asScala
-    results ++= columns.map(field => (field.getName, field.getType, field.getComment))
-    if (partitionColumns.nonEmpty) {
-      val partColumnInfo =
-        partitionColumns.map(field => (field.getName, field.getType, field.getComment))
-      results ++=
-        partColumnInfo ++
-          Seq(("# Partition Information", "", "")) ++
-          Seq((s"# ${output(0).name}", output(1).name, output(2).name)) ++
-          partColumnInfo
-    }
+    // There are two modes here:
+    // For metastore tables, create an output similar to Hive's.
+    // For other tables, delegate to DescribeCommand.
 
-    if (isExtended) {
-      results ++= Seq(("Detailed Table Information", table.hiveQlTable.getTTable.toString, ""))
-    }
+    // In the future, we will consolidate the two and simply report what the catalog reports.
+    sqlContext.sessionState.catalog.lookupRelation(tableId) match {
+      case table: MetastoreRelation =>
+        // Trying to mimic the format of Hive's output. But not exactly the same.
+        var results: Seq[(String, String, String)] = Nil
+
+        val columns: Seq[FieldSchema] = table.hiveQlTable.getCols.asScala
+        val partitionColumns: Seq[FieldSchema] = table.hiveQlTable.getPartCols.asScala
+        results ++= columns.map(field => (field.getName, field.getType, field.getComment))
+        if (partitionColumns.nonEmpty) {
+          val partColumnInfo =
+            partitionColumns.map(field => (field.getName, field.getType, field.getComment))
+          results ++=
+            partColumnInfo ++
+              Seq(("# Partition Information", "", "")) ++
+              Seq((s"# ${output(0).name}", output(1).name, output(2).name)) ++
+              partColumnInfo
+        }
+
+        if (isExtended) {
+          results ++= Seq(("Detailed Table Information", table.hiveQlTable.getTTable.toString, ""))
+        }
+
+        results.map { case (name, dataType, comment) =>
+          Row(name, dataType, comment)
+        }
 
-    results.map { case (name, dataType, comment) =>
-      Row(name, dataType, comment)
+      case o: LogicalPlan =>
+        DescribeCommand(tableId, output, isExtended).run(sqlContext)
     }
   }
 }