PROBLEM:

After restoring an RAC database to a standalone database through RMAN, we tried to drop the redolog groups belongs to thread 2. We hit the below error.

SQL> alter database drop logfile group 8;
alter database drop logfile group 8
*
ERROR at line 1:
ORA-00350: log 4 of instance DBATST2 (thread 2) needs to be archived
ORA-00312: online log 8 thread 2: '/u01/oracle/DBATST/onlinelog/redo08.log'

SOLUTION:

Before dropping, we need to clear the redolog using below command.

SQL> alter database clear unarchived logfile group 8;

Database altered.

Now try to drop again:

SQL> alter database drop logfile group 8;

Database altered.

Voilla..