What is FLASHBACK TABLE ? Explain with examples?

Purpose

Use the FLASHBACK TABLE statement to restore an earlier state of a table in the event of human or application error. The time in the past to which the table can be flashed back is dependent on the amount of undo data in the system. Also, Oracle Database cannot restore a table to an earlier state across any DDL operations that change the structure of the table.

Oracle strongly recommends that you run your database in Automatic Undo Management mode by setting the UNDO_MANAGEMENT initialization parameter to AUTO. In addition, set the UNDO_RETENTION initialization parameter to an interval large enough to include the oldest data you anticipate needing. For more information please refer to the documentation on the UNDO_MANAGEMENT and UNDO_RETENTION initialization parameters.

You cannot roll back a FLASHBACK TABLE statement. However, you can issue another FLASHBACK TABLE statement and specify a time just prior to the current time. Therefore, it is advisable to record the current SCN before issuing a FLASHBACK TABLE clause.

Prerequisites

To flash back a table to an earlier SCN or timestamp, you must have either the FLASHBACK object privilege on the table or the FLASHBACK ANY TABLE system privilege. In addition, you must have the SELECT, INSERT, DELETE, and ALTER object privileges on the table. Row movement must be enabled for all tables in the Flashback list. Please refer to row_movement_clause for information on enabling row movement.

To flash back a table to before a DROP TABLE operation, you need only the privileges necessary to drop the table.

Semantics

During an Oracle Flashback Table operation, Oracle Database acquires exclusive DML locks on all the tables specified in the Flashback list. These locks prevent any operations on the tables while they are reverting to their earlier state.

The Flashback Table operation is executed in a single transaction, regardless of the number of tables specified in the Flashback list. Either all of the tables revert to the earlier state or none of them do. If the Flashback Table operation fails on any table, then the entire statement fails.

At the completion of the Flashback Table operation, the data in table is consistent with table at the earlier time. However, FLASHBACK TABLE TO SCN or TIMESTAMP does not preserve rowids, and FLASHBACK TABLE TO BEFORE DROP does not recover referential constraints.

Oracle Database does not revert statistics associated with table to their earlier form. Indexes on table that exist currently are reverted and reflect the state of the table at the Flashback point. If the index exists now but did not yet exist at the Flashback point, then the database updates the index to reflect the state of the table at the Flashback point. However, indexes that were dropped during the interval between the Flashback point and the current time are not restored.

schema

Specify the schema containing the table. If you omit schema, then the database assumes the table is in your own schema.

table

Specify the name of one or more tables containing data you want to revert to an earlier version.

Restrictions on Flashing Back Tables

*

Flashback Table operations are not valid for the following type objects: tables that are part of a cluster, materialized views, Advanced Queuing (AQ) tables, static data dictionary tables, system tables, remote tables, object tables, nested tables, or individual table partitions or subpartitions.
*

The following DDL operations change the structure of a table, so that you cannot subsequently use the TO SCN or TO TIMESTAMP clause to flash the table back to a time preceding the operation: upgrading, moving, or truncating a table; adding a constraint to a table, adding a table to a cluster; modifying or dropping a column; adding, dropping, merging, splitting, coalescing, or truncating a partition or subpartition (with the exception of adding a range partition).


TO SCN Clause

Specify the system change number (SCN) corresponding to the point in time to which you want to return the table. The expr must evaluate to a number representing a valid SCN.

TO TIMESTAMP Clause

Specify a timestamp value corresponding to the point in time to which you want to return the table. The expr must evaluate to a valid timestamp in the past.

ENABLE | DISABLE TRIGGERS

By default, Oracle Database disables all enabled triggers defined on table during the Flashback Table operation and then reenables them after the Flashback Table operation is complete. Specify ENABLE TRIGGERS if you want to override this default behavior and keep the triggers enabled during the Flashback process.

This clause affects only those database triggers defined on table that are already enabled. To enable currently disabled triggers selectively, use the ALTER TABLE ... enable_disable_clause before you issue the FLASHBACK TABLE statement with the ENABLE TRIGGERS clause.

TO BEFORE DROP Clause

Use this clause to retrieve from the recycle bin a table that has been dropped, along with all possible dependent objects.

You can specify either the original user-specified name of the table or the system-generated name Oracle Database assigned to the object when it was dropped.

*

System-generated recycle bin object names are unique. Therefore, if you specify the system-generated name, then the database retrieves that specified object.

To see the contents of your recycle bin, query the USER_RECYCLEBIN data dictionary review. You can use the RECYCLEBIN synonym instead. The following two statements return the same rows:

SELECT * FROM RECYCLEBIN;
SELECT * FROM USER_RECYCLEBIN;


*

If you specify the user-specified name, and if the recycle bin contains more than one object of that name, then the database retrieves the object that was moved to the recycle bin most recently. If you want to retrieve an older version of the table, do one of these things:
o

Specify the system-generated recycle bin name of the table you want to retrieve.
o

Issue additional FLASHBACK TABLE ... TO BEFORE DROP statements until you retrieve the table you want.

Oracle Database attempts to preserve the original table name. If a new table of the same name has been created in the same schema since the original table was dropped, then the database returns an error unless you also specify the RENAME TO clause.

RENAME TO Clause

Use this clause to specify a new name for the table being retrieved from the recycle bin.

Notes on Flashing Back Dropped Tables

*

Oracle Database retrieves all indexes defined on the table retrieved from the recycle bin except for bitmap join indexes. (Bitmap join indexes are not put in the recycle bin during a DROP TABLE operation, so cannot be retrieved.)
*

The database also retrieves all triggers and constraints defined on the table except for referential integrity constraints that reference other tables.

The retrieved indexes, triggers, and constraints have recycle bin names. Therefore it is advisable to query the USER_RECYCLEBIN view before issuing a FLASHBACK TABLE ... TO BEFORE DROP statement so that you can rename the retrieved triggers and constraints to more usable names.
*

When you drop a table, all materialized view logs defined on the table are also dropped but are not placed in the recycle bin. Therefore, the materialized view logs cannot be flashed back along with the table.
*

When you drop a table, any indexes on the table are dropped and put into the recycle bin along with the table. If subsequent space pressures arise, then the database reclaims space from the recycle bin by first purging indexes. In this case, when you flash back the table, you may not get back all of the indexes that were defined on the table.
*

You cannot flash back a table if it has been purged, either by a user or by Oracle Database as a result of some space reclamation operation.

Examples

Restoring a Table to an Earlier State: Examples

The examples below create a new table, employees_demo, with row movement enabled, update values within the new table, and issue the FLASHBACK TABLE statement.

Create table employees_demo, with row movement enabled, from table employees of the sample hr schema:

CREATE TABLE employees_demo
ENABLE ROW MOVEMENT
AS SELECT * FROM employees;


As a benchmark, list those salaries less than 2500:

SELECT salary
FROM employees_demo
WHERE salary < 2500;

SALARY
----------
2400
2200
2100
2400
2200



Note:
To allow time for the SCN to propagate to the mapping table used by the FLASHBACK TABLE statement, wait a minimum of 5 minutes prior to issuing the following statement. This wait would not be necessary if a previously existing table were being used in this example.

Issue a 10% salary increase to those employees earning less than 2500:

UPDATE employees_demo
SET salary = salary * 1.1
WHERE salary < 2500;

5 rows updated.
COMMIT;


As a second benchmark, list those salaries that remain less than 2500 following the 10% increase:

SELECT salary
FROM employees_demo
WHERE salary < 2500;

SALARY
----------
2420
2310
2420


Restore the table employees_demo to its state 1 minute prior to the current system time:

FLASHBACK TABLE employees_demo
TO TIMESTAMP (SYSTIMESTAMP - INTERVAL '1' minute);


List those salaries less than 2500. After the FLASHBACK TABLE statement issued above, this list should match the list in the first benchmark.

SELECT salary
FROM employees_demo
WHERE salary < 2500;

SALARY
----------
2400
2200
2100
2400
2200


Retrieving a Dropped Table: Example

If you accidentally drop the hr.employees table and wish to retrieve it, issue the following statement:

FLASHBACK TABLE employees TO BEFORE DROP;


If another employees table has been created in the hr schema, use the RENAME TO clause to rename the retrieved table:

FLASHBACK TABLE employees TO BEFORE DROP RENAME TO employees_old;


If you know that the employees table has been dropped multiple times, and you wish to retrieve the oldest version, query the USER_RECYLEBIN table to determine the system-generated name, and then use that name in the FLASHBACK TABLE statement. (System-generated names in your database will differ from those shown here.)

SELECT object_name, droptime FROM user_recyclebin
WHERE original_name = 'employees';

OBJECT_NAME DROPTIME
------------------------------ -------------------
RB$$45703$TABLE$0 2003-06-03:15:26:39
RB$$45704$TABLE$0 2003-06-12:12:27:27
RB$$45705$TABLE$0 2003-07-08:09:28:01

Flashback Dropped Table
Flashback Table From The Recycle Bin Using The Object Name FLASHBACK TABLE TO BEFORE DROP
{RENAME TO };
CREATE TABLE test (
testcol VARCHAR2(20));

ALTER TABLE test
ADD CONSTRAINT pk_test
PRIMARY KEY (testcol)
USING INDEX
PCTFREE 0;

INSERT INTO test VALUES ('ABC');
INSERT INTO test VALUES ('DEF');
COMMIT;

SELECT * FROM test;

DROP TABLE test;

SELECT object_name, original_name
FROM recyclebin;

FLASHBACK TABLE "BIN$4/V75AlOSDqCJ4fbHLAtpQ==$0"
TO BEFORE DROP;

SELECT * FROM test;

SELECT constraint_name, constraint_type
FROM user_constraints
ORDER BY table_name;
Flashback Table From The Recycle Bin Using The Original Name FLASHBACK TABLE TO BEFORE DROP
{RENAME TO };
CREATE TABLE test (
testcol VARCHAR2(20));

INSERT INTO test VALUES ('ABC');

COMMIT;

SELECT * FROM test;

DROP TABLE test;

CREATE TABLE test (
testcol VARCHAR2(20));

INSERT INTO test VALUES ('DEF');

COMMIT;

SELECT * FROM test;

DROP TABLE test;

CREATE TABLE test (
testcol VARCHAR2(20));

INSERT INTO test VALUES ('GHI');

COMMIT;

SELECT * FROM test;

DROP TABLE test;

SELECT object_name, original_name, droptime
FROM recyclebin;

FLASHBACK TABLE test TO BEFORE DROP;

FLASHBACK TABLE test TO BEFORE DROP
RENAME TO test2;

FLASHBACK TABLE test TO BEFORE DROP
RENAME TO test1;

SELECT * FROM test;

SELECT * FROM test2;

SELECT * FROM test1;

Flashback To SCN

Flashback to identified SCN demo FLASHBACK TABLE TO SCN ;
CREATE TABLE test (
testcol VARCHAR2(20));

SELECT current_scn
FROM gv$database;

INSERT INTO test VALUES ('ABC');
COMMIT;

SELECT current_scn
FROM gv$database;

INSERT INTO test VALUES ('DEF');
COMMIT;

SELECT current_scn
FROM gv$database;

INSERT INTO test VALUES ('GHI');
COMMIT;

SELECT current_scn
FROM gv$database;

SELECT * FROM test;

SELECT current_scn
FROM gv$database;

ALTER TABLE test ENABLE ROW MOVEMENT;

SELECT * FROM test;

FLASHBACK TABLE test TO SCN 1833265;

Flashback To Timestamp

Flashback to identified timestamp demo FLASHBACK TABLE TO TIMESTAMP ;
CREATE TABLE test (
testcol VARCHAR2(20));

SELECT systimestamp
FROM dual;

INSERT INTO test VALUES ('ABC');
COMMIT;

SELECT systimestamp
FROM dual;

INSERT INTO test VALUES ('DEF');
COMMIT;

SELECT systimestamp
FROM dual;

INSERT INTO test VALUES ('GHI');
COMMIT;

SELECT systimestamp
FROM dual;

SELECT * FROM test;

SELECT systimestamp
FROM dual;

ALTER TABLE test ENABLE ROW MOVEMENT;

SELECT * FROM test;

FLASHBACK TABLE test TO TIMESTAMP
TO_TIMESTAMP('14-JUN-05 10.14.34.269 PM');

SELECT * FROM test;