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Undo the past with Oracle's Flashback Technology

Pankaj Chandiramani's picture

Flashback Recovery is a new enhancement to the 10g database for the DBA's toolkit. Effectively, it's an "Oh shit!" protection mechanism for DBAs as it reduces recovery time from hours to minutes. Ask any DBA about the main cause of application outage - other than hardware failure - and the answer will be "human error". Such errors can result in logical data corruption and can bring down the complete system. Part of being human is making mistakes. Without advance planning, these errors are extremely difficult to avoid, and can be very difficult to recover from. Typical user-errors may include the accidental deletion of valuable data, deleting the wrong data, or dropping the wrong table.

So, the big question is: How can we protect a database from human error?

The answer is a technology called "Flashback" - a major enhancement in 10g, as it revolutionizes recovery by working just on the changed data.

Flashback provides:

  • An effective way to recover from complex human errors
  • Faster database point-in-time recovery
  • Simplified management and administration
  • Little performance overhead

The performance overhead of enabling Flashback Database is less than 2%. While you may not be willing to sacrifice any performance overhead for your production database, think about the trade-off. If you could recover the database in minutes instead of hours, saving your company millions of dollars in lost revenue, wouldn't you be willing to give 2% of the resources to Flashback Database?

Below is a chat to show how quickly Flashback can recover your database.

Architecture:

There are some basic prerequisites for Flashback recovery:

  • The database should be in Archive Log Mode. Issue the alter database archivelog command when the database is mounted.

  • Some of the parameters should be set as follows:

    • DB_FLASHBACK_RETENTION_TARGET: This specifies the time limit for deleted data to be maintained in the database. E.g. alter system set DB_FLASHBACK_RETENTION_TARGET=4320, i.e. 72 hours

    • DB_RECOVERY_FILE_DEST_SIZE: This specifies the maximum data that can be retained. E.g. alter system set DB_RECOVERY_FILE_DEST_SIZE=536870912, i.e. 512 MB

    • DB_RECOVERY_FILE_DEST: This specifies the destination for the flashback data. E.g. alter system set DB_RECOVERY_FILE_DEST='/u02/fra';

Types of Flashback recoveries

There are six basic types of Flashback recovery, discussed below in detail:

  • Flashback Query
  • Flashback Version Query
  • Flashback Transaction Query
  • Flashback Table
  • Flashback Drop (Recycle Bin)
  • Flashback Database

Flashback Query:

You can perform a Flashback Query using a SELECT statement with an AS OF clause. You can use a Flashback Query to retrieve data as it existed at some time in the past. The query explicitly references a past time using a timestamp or SCN. It returns committed data that was current at that point in time.

Example:

This example uses a Flashback Query to examine the state of a table at a specified time in the past. Suppose, for instance, that a DBA discovers at 12:30 PM that data for employee JON has been deleted from the employee table, and the DBA knows that at 9:30AM the data for JON was correctly stored in the database. The DBA can use a Flashback Query to examine the contents of the table at 9:30, to find out what data has been lost. If appropriate, the DBA can then re-insert the lost data in the database.

The following query retrieves the state of the employee record for JOHN at 9:30AM, April 4, 2006:

SQL> SELECT * FROM employee AS OF TIMESTAMP 
   TO_TIMESTAMP('2006-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS')
   WHERE name = 'JON'; 

This update then restores John's information to the employee table:

SQL> INSERT INTO employee 
    (SELECT * FROM employee AS OF TIMESTAMP 
     TO_TIMESTAMP('2006-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS')
     WHERE name = 'JON'); 

Flashback Query Functions

The TIMESTAMP_TO_SCN and SCN_TO_TIMESTAMP functions have been added to SQL and PL/SQL to simplify flashback operations:

DECLARE
  l_scn        NUMBER;
  l_timestamp  TIMESTAMP;
BEGIN
  l_scn       := TIMESTAMP_TO_SCN(SYSTIMESTAMP - 1/24);
  l_timestamp := SCN_TO_TIMESTAMP(l_scn);
END;
/

Flashback Version Query

The Flashback Version Query is a powerful tool for the DBA to run analysis and answer the question, "How did this happen?".

Not only can the DBA run a manual analysis, but this is a powerful tool for the application's developer as well. You can build customized applications for auditing purposes. Now everyone really is accountable for his or her actions. Various elements for this are shown below:

VERSIONS_XID - The transaction id that created this version of the row
VERSIONS_OPERATION - The action that created this version of the row (such as delete, insert, and update)
VERSIONS_STARTSCN - The SCN in which this row version first occurred
VERSIONS_ENDSCN - The SCN in which this row version was changed.

For example: we use the Dept table in Scott schema and update dept 10 to 11, then 12, and then 13 (with a commit after every update). Thus we have done three updates:

SQL> select versions_starttime, versions_endtime, versions_xid, versions_operation, 
dept 
from Dept versions between timestamp minvalue and maxvalue
order by VERSIONS_STARTTIME

VERSIONS_STARTTIME VERSIONS_ENDTIME VERSIONS_XID V Dept
---------------------- ---------------------- ---------------- - ----------
01-DEC-06 03.57.12 PM 01-DEC-06 03.57.30 PM 0002002800000C61 I 10
01-DEC-06 03.57.30 PM 01-DEC-06 03.57.39 PM 000A000A00000029 U 11
01-DEC-06 03.57.39 PM 01-DEC-06 03.57.55 PM 000A000B00000029 U 12

We can automatically generate the undo statement using:

SQL> SELECT UNDO_SQL FROM FLASHBACK_TRANSACTION_QUERY
WHERE XID = '000A000A00000029';
- output: update "SCOTT"."Dept" set "Dept" = '12' where ROWID = 'AAAMicAAEAAAAA/AAA'; 

Flashback Transaction Query

A Flashback Transaction Query is a query on a view called FLASHBACK_TRANSACTION_QUERY. You use a Flashback Transaction Query to obtain transaction information, including SQL code that you can use to undo each of the changes made by the transaction. It uses the XID from above and shows the complete transaction steps for that XID. For example:

SQL> SELECT  xid, start_scn START, commit_scn COMMIT, 
        operation OP, logon_user USER, undo_sql FROM flashback_transaction_query
        WHERE xid = HEXTORAW('000200030000002D');

XID               START   COMMIT  OP       USER   UNDO_SQL
----------------  -----   ------  --       ----   ---------------------------
000200030000002D  195243  195244  DELETE   HR     insert into "HR"."EMP" 
("EMPNO","EMPNAME","SALARY") values ('111','Mike','655');

000200030000002D  195243  195244  INSERT   HR     delete from "HR"."DEPT" 
where ROWID = 'AAAKD4AABAAAJ3BAAB';

000200030000002D  195243  195244  UPDATE   HR     update "HR"."EMP" 
set "SALARY" = '555' where ROWID = 'AAAKD2AABAAAJ29AAA';

000200030000002D  195243  113565  BEGIN  HR

4 rows selected.

Flashback Table

Just as the flashback query helps to retrieve rows of a table, FLASHBACK TABLE helps to restore the state of a table to a certain point in time - even if a table structure change has occurred since then. The following simple command will take us to the table state at the specified timestamp:

SQL> FLASHBACK TABLE Employee TO 
           TIMESTAMP ('13-SEP-06 8:50:58','DD-MON-YY HH24: MI: SS');

Flashback Drop (Recycle Bin)

The accidental dropping of objects has always been a problem for users and DBAs alike. Users soon realize their mistake, but by then it's too late - and until now there has been no easy way to recover those dropped tables, indexes, constraints, triggers, etc.

Flashback Drop provides a safety net when dropping objects in Oracle Database 10g. When a user drops a table, Oracle automatically places it into the Recycle Bin.

The Recycle Bin

The Recycle Bin is a virtual container for all dropped objects. Under its lid, the objects occupy the same space as when they were created. If table EMP was created in the USERS tablespace, the dropped table EMP remains in the
USERS tablespace. Dropped tables and any associated objects such as indexes, constraints, nested tables, and other dependent objects are not moved; they are simply renamed with the prefix BIN$$. You can continue to access the data in a dropped table, or even use Flashback Query on it.

SQL> FLASHBACK TABLE dept_test TO BEFORE DROP;

Flashback Database

We have been talking about object recovery. But what if something happens to the whole database? This recovery quickly rewinds the complete database to a previous time, to correct any logical data corruption. This can be used with both RMAN & SQL*Plus.
Some of the options are:

FLASHBACK DATABASE TO TIMESTAMP SYSDATE-(1/24/12);
FLASHBACK DATABASE TO TIMESTAMP my_date;
FLASHBACK DATABASE TO BEFORE TIMESTAMP my_date;
FLASHBACK DATABASE TO SCN my_scn

When the system comes back with FLASHBACK COMPLETE, open the database with Resetlogs.

For example:

-- Create a dummy table.
CREATE TABLE flashback_database_test (id  NUMBER(10)

--Flashback 5 Minutes
CONN sys/password AS SYSDBA
SHUTDOWN IMMEDIATE
STARTUP MOUNT EXCLUSIVE
FLASHBACK DATABASE TO TIMESTAMP SYSDATE-(1/24/12);
ALTER DATABASE OPEN RESETLOGS;

-- Check that the table is gone.
DESC flashback_database_test

Conclusion:

Flashback is an insurance feature. Regard it as another tool on your belt, rather than as a luxury. That said, it does not allow us to be careless with data simply because we have the ability to recover it easily.