How to Enable Flashback Database

To enable flashback database the following operations is needed.

1)Configure the Database in archivelog mode.
To change archiving read, Change Archival Mode

2)Configure Flash Recovery Area.
To configure flash recovery area,
Set up Flash Recovery Area

3)Clean Shutdown and mount the database.

Alter Database Flashback ON;

Before running command you can check whether flashback was actually On or not.

select flashback_on from v$database;

Steps:
------
SQL> alter database flashback ON;
Database altered.

SQL> select flashback_on from v$database;
FLASHBACK_ON
------------------
YES

4)Open the database and optionally you can set DB_FLASHBACK_RETENTION_TARGET to the length of the desired flashback window in minutes. By default it is 1 day(1440 minutes).

To make it 3 days
SQL> ALTER SYSTEM SET DB_FLASHBACK_RETENTION_TARGET=4320;
SQL> show parameter DB_FLASHBACK_RETENTION_TARGET

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_flashback_retention_target integer 4320

However you can disable Flashback Database for a tablespace.Then you must take its datafiles offline before running FLASHBACK DATABASE.

Like,

SQL> select file_name,file_id from dba_data_files where tablespace_name='TEST';
FILE_NAME FILE_ID
------------------------------ ----------
/oradata2/1.dbf 5

SQL> alter database datafile 5 offline;
Database altered.

SQL> ALTER TABLESPACE test flashback off;
Tablespace altered.

SQL> recover datafile 5;
Media recovery complete.

SQL> alter database datafile 5 online;
Database altered.

To disable flashback feature simply issue,
SQL>ALTER DATABASE FLASHBACK OFF;
Database altered.