[SPARK-18464][SQL] support old table which doesn't store schema in metastore

sql/core/src/main/scala/org/apache/spark/sql/execution/command/tables.scala

@@ -431,7 +431,13 @@ case class DescribeTableCommand(
       describeSchema(catalog.lookupRelation(table).schema, result)
     } else {
       val metadata = catalog.getTableMetadata(table)
-      describeSchema(metadata.schema, result)
+      if (metadata.schema.isEmpty) {
+        // In older version(prior to 2.1) of Spark, the table schema can be empty and should be
+        // inferred at runtime. We should still support it.
+        describeSchema(catalog.lookupRelation(metadata.identifier).schema, result)
+      } else {
+        describeSchema(metadata.schema, result)
+      }
 
       describePartitionInfo(metadata, result)
 

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

@@ -1023,6 +1023,11 @@ object HiveExternalCatalog {
       // After SPARK-6024, we removed this flag.
       // Although we are not using `spark.sql.sources.schema` any more, we need to still support.
       DataType.fromJson(schema.get).asInstanceOf[StructType]
+    } else if (props.filterKeys(_.startsWith(DATASOURCE_SCHEMA_PREFIX)).isEmpty) {
+      // If there is no schema information in table properties, it means the schema of this table
+      // was empty when saving into metastore, which is possible in older version(prior to 2.1) of
+      // Spark. We should respect it.
+      new StructType()
     } else {
       val numSchemaParts = props.get(DATASOURCE_SCHEMA_NUMPARTS)
       if (numSchemaParts.isDefined) {

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

@@ -64,7 +64,9 @@ private[hive] class HiveMetastoreCatalog(sparkSession: SparkSession) extends Log
         val dataSource =
           DataSource(
             sparkSession,
-            userSpecifiedSchema = Some(table.schema),
+            // In older version(prior to 2.1) of Spark, the table schema can be empty and should be
+            // inferred at runtime. We should still support it.
+            userSpecifiedSchema = if (table.schema.isEmpty) None else Some(table.schema),
             partitionColumns = table.partitionColumnNames,
             bucketSpec = table.bucketSpec,
             className = table.provider.get,

sql/hive/src/test/scala/org/apache/spark/sql/hive/MetastoreDataSourcesSuite.scala

@@ -1371,4 +1371,26 @@ class MetastoreDataSourcesSuite extends QueryTest with SQLTestUtils with TestHiv
       }
     }
   }
+
+  test("SPARK-18464: support old table which doesn't store schema in table properties") {
+    withTable("old") {
+      withTempPath { path =>
+        Seq(1 -> "a").toDF("i", "j").write.parquet(path.getAbsolutePath)
+        val tableDesc = CatalogTable(
+          identifier = TableIdentifier("old", Some("default")),
+          tableType = CatalogTableType.EXTERNAL,
+          storage = CatalogStorageFormat.empty.copy(
+            properties = Map("path" -> path.getAbsolutePath)
+          ),
+          schema = new StructType(),
+          properties = Map(
+            HiveExternalCatalog.DATASOURCE_PROVIDER -> "parquet"))
+        hiveClient.createTable(tableDesc, ignoreIfExists = false)
+
+        checkAnswer(spark.table("old"), Row(1, "a"))
+
+        checkAnswer(sql("DESC old"), Row("i", "int", null) :: Row("j", "string", null) :: Nil)
+      }
+    }
+  }
 }