[table editor] allow selecting physical table (#6046)

* [table editor] allow selecting physical table

* Using classes for padding

superset/views/datasource.py

@@ -8,6 +8,7 @@ from flask_babel import gettext as __
 
 from superset import appbuilder, db
 from superset.connectors.connector_registry import ConnectorRegistry
+from superset.models.core import Database
 from .base import BaseSupersetView, check_ownership, json_error_response
 
 
@@ -42,9 +43,24 @@ class Datasource(BaseSupersetView):
     @has_access_api
     def external_metadata(self, datasource_type=None, datasource_id=None):
         """Gets column info from the source system"""
-        orm_datasource = ConnectorRegistry.get_datasource(
-            datasource_type, datasource_id, db.session)
-        return self.json_response(orm_datasource.external_metadata())
+        if datasource_type == 'druid':
+            datasource = ConnectorRegistry.get_datasource(
+                datasource_type, datasource_id, db.session)
+        elif datasource_type == 'table':
+            database = (
+                db.session
+                .query(Database)
+                .filter_by(id=request.args.get('db_id'))
+                .one()
+            )
+            Table = ConnectorRegistry.sources['table']
+            datasource = Table(
+                database=database,
+                table_name=request.args.get('table_name'),
+                schema=request.args.get('schema') or None,
+            )
+        external_metadata = datasource.external_metadata()
+        return self.json_response(external_metadata)
 
 
 appbuilder.add_view_no_menu(Datasource)