Add condition to try query external tables if none found in redshift

Author: danieldiamond

data_diff/sqeleton/databases/base.py

@@ -339,6 +339,8 @@ def select_table_schema(self, path: DbPath) -> str:
 
     def query_table_schema(self, path: DbPath) -> Dict[str, tuple]:
         rows = self.query(self.select_table_schema(path), list)
+        if not rows and self.name == 'Redshift':
+            rows = self.query(self.select_external_table_schema(path), list)
         if not rows:
             raise RuntimeError(f"{self.name}: Table '{'.'.join(path)}' does not exist, or has no columns")