To check if Flashback is enable
select log_mode,flashback_on from v$database;
LOG_MODE FLASHBACK_ON
------------ ------------------
ARCHIVELOG NO
To Enable Flashback
SQL> alter database flashback on;
Database altered.
SQL> select log_mode,flashback_on from v$database;
LOG_MODE FLASHBACK_ON
------------ ------------------
ARCHIVELOG YES
To Disable Flashback
SQL> alter database flashback off;
Database altered.
SQL> select log_mode,flashback_on from v$database;
LOG_MODE FLASHBACK_ON
------------ ------------------
ARCHIVELOG NO
