[Oracle DataBase Server 12.2 Installation on Oracle Linux 7.4]: Enable FLASH BACK Mode

FlashBack is useful when you need to roll back changes or view the previous state of objects in the database. As a consequence, server load increases because additional information must be stored.

$ sqlplus / as sysdba

SQL> shutdown immediate;
SQL> startup mount exclusive;
SQL> alter database flashback on;
SQL> alter database open;

SQL> select flashback_on from v$database;

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

UNDO_RETENTION - (with FLASHBACK enabled) determines the minimum time in seconds for which a change in the database can be undone (viewed). The data will be stored in UNDO_TABLESPACE (you must ensure sufficient tablespace size) and overwritten as needed, ensuring the minimum value specified in UNDO_RETENTION. Not supported for LOB.

Set the UNDO_RETENTION parameter to 30 minutes

SQL> alter system set UNDO_RETENTION = 1800;
SQL> alter tablespace UNDO RETENTION GUARANTEE;

SQL> show parameter UNDO_RETENTION

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_retention                       integer     1800

SQL> quit