Tales From A Lazy Fat DBA

Diffrence between “/ as sysdba” and “sys/pswrd as sysdba”

— sqlplus “/as sysdba” it mean is you use OS authorization and your user must member of dba(ORA_DBA) OS group,else operation will fail.
— sqlplus sys/pass@sid as sysdba it mean is you use passwordfile authorization .And in this case you need properly configure this file also need set remote_login_passwordfile= EXCLUSIVE or SHARED.

And @sid (sqlplus sys/pass@sid as sysdba) also need listener to be up, can be done either in the server or client side based on the entry in your tnsnames.ora otherwise you’ll receive error message on the SQL terminal the moment you try logging – ORA-12541: TNS:no listener
sqlplus / as sysdba does not require listener to be up but has to be done in the server side.