sqlalchemy: DatabaseError: (cx_Oracle.DatabaseError) ORA-01219: database not open: queries allowed on fixed tables/views only

Describe the bug

I am trying to query Oracle standby database which is in mount mode (not opened). And when querying the database I am getting

DatabaseError: (cx_Oracle.DatabaseError) ORA-01219: database not open: queries allowed on fixed tables/views only
[SQL: select value from nls_session_parameters where parameter = 'NLS_NUMERIC_CHARACTERS']
(Background on this error at: https://sqlalche.me/e/14/4xp6)
 

To Reproduce

DIALECT = 'oracle'
SQL_DRIVER = 'cx_oracle'
USERNAME = 'sys' #enter your username
PASSWORD = 'syspassword' #enter your password
CONN_STR='tns_alias'
ENGINE_PATH_WIN_AUTH = DIALECT + '+' + SQL_DRIVER + '://' + USERNAME + ':' + PASSWORD +'@' + CONN_STR + '?mode='+str(cx_Oracle.SYSDBA) + '&encoding=UTF-8&nencoding=UTF-8'

import pandas as pd
from sqlalchemy.engine import create_engine
conn = create_engine(ENGINE_PATH_WIN_AUTH)
pd.read_sql("select instance_name from v$instance", con=conn)

Error

<skipped>

File c:\python38\lib\site-packages\sqlalchemy\dialects\oracle\cx_oracle.py:1038, in OracleDialect_cx_oracle.initialize(self, connection)
   1035 if self._is_oracle_8:
   1036     self.supports_unicode_binds = False
-> 1038 self._detect_decimal_char(connection)

File c:\python38\lib\site-packages\sqlalchemy\dialects\oracle\cx_oracle.py:1107, in OracleDialect_cx_oracle._detect_decimal_char(self, connection)
   1100 def _detect_decimal_char(self, connection):
   1101     # we have the option to change this setting upon connect,
   1102     # or just look at what it is upon connect and convert.
   1103     # to minimize the chance of interference with changes to
   1104     # NLS_TERRITORY or formatting behavior of the DB, we opt
   1105     # to just look at it
-> 1107     self._decimal_char = connection.exec_driver_sql(
   1108         "select value from nls_session_parameters "
   1109         "where parameter = 'NLS_NUMERIC_CHARACTERS'"
   1110     ).scalar()[0]
   1111     if self._decimal_char != ".":
   1112         _detect_decimal = self._detect_decimal

<skipped>

DatabaseError: (cx_Oracle.DatabaseError) ORA-01219: database not open: queries allowed on fixed tables/views only
[SQL: select value from nls_session_parameters where parameter = 'NLS_NUMERIC_CHARACTERS']
(Background on this error at: https://sqlalche.me/e/14/4xp6)

Versions

• OS: MS Windows 10
• Python: 3.8
• SQLAlchemy: 1.4.42
• Database: Oracle 12.2
• DBAPI (eg: psycopg, cx_oracle, mysqlclient): cx_oracle

Additional context

Use dynamic view gv$nls_parameters instead of nls_sessions_parameters when trying to set decimal separator. It will work with not opened (but mounted) database as well.