How to enable and disable flashback

Oracle Database - Enterprise Edition - version 11.2.0.4.0

Enable flashback

Make sure the database is in archive log mode.

Refer: How to enable and disable archive log mode

To enable flashback we need to set two parameters: DB_RECOVERY_FILE_DEST and DB_RECOVERY_FILE_DEST_SIZE.

SQL> alter system set db_recovery_file_dest='/home/oracle/app/oracle';

System altered.

SQL> alter system set db_recovery_file_dest_size=10g;

System altered.

SQL> show parameter db_recovery_file

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest string /home/oracle/app/oracle
db_recovery_file_dest_size big integer 10G

SQL> select flashback_on from v$database;

FLASHBACK_ON
------------------
NO

SQL> alter database flashback on;

Database altered.

SQL> select flashback_on from v$database;

FLASHBACK_ON
------------------
YES

Disable flashback

SQL> alter database flashback off;

Database altered.

SQL> select flashback_on from v$database;

FLASHBACK_ON
------------------
NO

If you are in 10g, need to enable/disable the flashback mode in mount stage.
shutdown immediate
startup mount
alter database flashback off;
alter database open;