fix: datatype tracking issue on virtual dataset (#20088)

* Fix datatype tracking issue on virtual dataset

* fix pytest issue on postgresql

superset/db_engine_specs/postgres.py

@@ -21,6 +21,7 @@ from datetime import datetime
 from typing import Any, Dict, List, Optional, Pattern, Tuple, TYPE_CHECKING
 
 from flask_babel import gettext as __
+from psycopg2.extensions import binary_types, string_types
 from sqlalchemy.dialects.postgresql import ARRAY, DOUBLE_PRECISION, ENUM, JSON
 from sqlalchemy.dialects.postgresql.base import PGInspector
 from sqlalchemy.types import String
@@ -287,6 +288,14 @@ class PostgresEngineSpec(PostgresBaseEngineSpec, BasicParametersMixin):
             native_type, column_type_mappings=column_type_mappings
         )
 
+    @classmethod
+    def get_datatype(cls, type_code: Any) -> Optional[str]:
+        types = binary_types.copy()
+        types.update(string_types)
+        if type_code in types:
+            return types[type_code].name
+        return None
+
     @classmethod
     def get_cancel_query_id(cls, cursor: Any, query: Query) -> Optional[str]:
         """