Flashback Table in Oracle Explained With Examples

Flashback table restores the existing tables to earlier versions using timestamp and SCN number. It is available in and after Oracle 10g version.

It also helps to retrieve removed tables from the database, dropped using DROP and TRUNCATE commands.

Tables altered using DDL statements can also not be restored to a prior state with flashback operation.

One thing you must remember about flashback operation is that undo of data is controlled by UNDO_RETENTION parameter. In simpler words, if undo data is available, then only tables can be restored back.

Once you trigger FLASHBACK TABLE statement on any table, it cannot be rolled back. Best practice is to record the current SCN number before issuing a FLASHBACK TABLE statement.

You must enable row movement during the creation of the table to carry out flashback operation on it. Row movement can also be enabled using ALTER command later on.

ALTER TABLE employees ENABLE ROW MOVEMENT;

Syntax

FLASHBACK TABLE table_name
[TO BEFORE DROP] |
[TO TIMESTAMP time_stamp] |
[TO SCN scn_number] |
[ENABLE TRIGGERS | DISABLE TRIGGERS];

Parameters Explanation

table_name — Name of the table to restore

time_stamp — User defined timestamp

scn_number — Oracle SCN (Service Change Number) number

Restoring Table to Earlier State

The below example shows FLASHBACK TABLE command used to restore the table to an earlier state.

Create table emp_temp from table employees whose department number is 10.

CREATE TABLE emp_temp
SELECT * FROM employees WHERE dept_no = 10;

Fetching the data from emp_temp retrieves the list of employees belonging to department number 10.

SELECT dept_no, salary FROM emp_temp;

Now, delete the employees with salary less than 2000 from the emp_temp table using below query.

DELETE FROM emp_temp WHERE salary < 2000;

This will leave only 2 records in the emp_temp table.

With below query, we can restore table emp_temp to 2 minutes prior to current time using flashback operation

FLASHBACK TABLE emp_temp
TO TIMESTAMP (SYSTIMESTAMP – INTERVAL '2' MINUTE);

Querying table emp_temp again will retrieve the original results

Another way of restoring table emp_temp to particular timestamp is as follows

FLASHBACK TABLE emp_temp
TO TIMESTAMP TO_TIMESTAMP('2018-09-19 13:34:12', 'YYYY-MM-DD HH24:MI:SS');

Retrieve Table Using SCN

You can also retrieve the older version of the table using SCN number.

Let us continue with the above example TO TIMESTAMP

In order to query 2 minutes, the prior version of table emp_temp using SCN number, below query can be used

FLASHBACK TABLE emp_temp TO SCN (3187302511937);

This SCN number is of 2 minutes prior state of table emp_temp.

So the output after flashback operation while querying emp_temp table will be

Retrieving a Dropped Table

Suppose you had dropped table employees accidentally in a hurry to go home by firing a below query

DROP TABLE employees;

To retrieve the table employees back, use TO BEFORE DROP clause of FLASHBACK TABLE.

FLASHBACK TABLE employees TO BEFORE DROP;

The retrieved employees table can also be renamed to emp with the below query

FLASHBACK TABLE employees TO BEFORE DROP RENAME TO emp;

ENABLE or DISABLE TRIGGERS

By default Oracle database disable triggers during the flashback. These triggers are enabled when flashback operation is completed.

You need to specify ENABLE TRIGGERS to keep the triggers enabled during flashback operation.

FLASHBACK TABLE emp_temp TO SCN (3187302511937) ENABLE TRIGGERS;

However, the triggers that are already disabled before flashback operation cannot be enabled using this option.

DISABLE TRIGGERS acts the same as a default behaviour in flashback operation.

RECYCLEBIN and USER_RECYCLEBIN

USER_RECYCLEBIN is the view from which you can retrieve the tables that are dropped or truncated.

SELECT * FROM user_recyclebin;

RECYCLEBIN is the synonym of USER_RECYCLEBIN view.

TO SCN Clause

The table can be returned to a point in time of mentioned SCN number using TO SCN clause.

SCN number can be retrieved from the v$database table using below query

SELECT current_scn FROM v$database;

You can also convert a timestamp to SCN number and vice versa using SCN_TO_TIMESTAMP and TIMESTAMP_TO_SCN functions

SELECT TIMESTAMP_TO_SCN(SYSTIMESTAMP – INTERVAL '2' MINUTE) FROM dual;

SCN_TO_TIMESTAMP(3187302511937) will retrieve the timestamp of this SCN number.

TO TIMESTAMP Clause

In order to restore the table back to particular timestamp, use TO TIMESTAMP clause.

TO BEFORE DROP Clause

The clause TO BEFORE DROP retrieves back the dropped table.

However, the tables dropped using PURGE option cannot be retrieved back.

TO BEFORE DROP clause is also unable to recover tables dropped TRUNCATE TABLE command.

RENAME TO Clause

RENAME TO clause retrieves the table from recycle bin with a new name during flashback operation.

Retrieve Table After TRUNCATE TABLE

Tables dropped by TRUNCATE TABLE can be restored using TO TIMESTAMP and TO SCN clause. Recovery of lost data is not possible in this case.