Pankaj Chandiramani's Blog

FlashBack Recovery is a new enhancement in 10g for the toolkit of a DBA.
Effectively, it's a "oh shit!" proctection mechanism for DBAs as it reduces recovery time from hours to minutes. If we ask any DBA abt the main causes of application outage other than hardware failure, the answer will be "Human Errors". Such errors can result in "logical" data corruption & can bring bown the complete system. Part of being human is making mistakes. But these errors are extremely difficult to avoid and can be particularly difficult to recover from without advance planning. Typical user-errors may include accidental deletion of valuable data, deleting the wrong data, and dropping the wrong table.

So the question comes.......How to guard against Human error ?
And the answer for this comes in a techonology called "FalshBack", which is a major enhancement in 10g as it revolutionizes recovery by operating just on the changed data. Thus time it takes to recover the error is now equal to the same amount of time it took to make the mistake.

Flashback Database provides:

• 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, would you then give 2% of the resources to Flashback Database?

Architecture

There are some basic pre-requesits for FlashBack recovery , these are :

• Database should be in ArchiveLog Mode :- On mount state use :alter database archivelog;
• Some of the parameters should be set as below :

1. DB_FLASHBACK_RETENTION_TARGET - This specifies the timelimit for deleted data to be maintained in database
Eg alter system set DB_FLASHBACK_RETENTION_TARGET=4320 , ie 72hrs

2. DB_RECOVERY_FILE_DEST_SIZE: This specifies the max data that can be retained ,
Eg alter system set DB_RECOVERY_FILE_DEST_SIZE=536870912 , ie 512Mb

3. DB_RECOVERY_FILE_DEST : This specifies the destination for the flashback data ,
Eg alter system set DB_RECOVERY_FILE_DEST='/u02/fra';


Types Of FlashBack Recoveries
There are basic 7 types are FlashBack recoveries, these are discussed below in details

1. Flashback Query
2. Flashback Version Query
3. Flashback Transaction Query
4. Flashback Table
5. Flashback Drop (Recycle Bin)
6. Flashback Database
7. Flashback Query Functions

1)Flashback Query :
You perform a Flashback Query using a SELECT statement with an AS OF clause. You 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 previous time. Suppose, for instance, that a DBA discovers at 12:30 PM that data for employee JOHN had been deleted from the employee table, and the DBA knows that at 9:30AM the data for JOHN was correctly stored in the database. The DBA can use a Flashback Query to examine the contents of the table at 9:30, to findout what data had 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, 2003:

SQL> SELECT * FROM employee AS OF TIMESTAMP
TO_TIMESTAMP('2003-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS')
WHERE name = 'JOHN';

This update then restores John's information to the employee table:
SQL>INSERT INTO employee (SELECT * FROM employee AS OF TIMESTAMP
TO_TIMESTAMP('2003-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS')
WHERE name = 'JOHN');

2)Flashback Version Query
The Flashback Versions 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 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.

Eg : we use the Dept table in Scott schema & update dept 10 to 11 & then 12 & then 13(with commit on every update).Thus we have run 3 updates
so a query like
SQL>select versions_starttime, versions_endtime, versions_xid, versions_operation, Dept from Dept versions between timestamp minvalue and maxvalue
order by VERSIONS_STARTTIME

will return something like :

VERSIONS_STARTTIME VERSIONS_ENDTIME VERSIONS_XID V Dept
---------------------- ---------------------- ---------------- - ----------
01-DEC-03 03.57.12 PM 01-DEC-03 03.57.30 PM 0002002800000C61 I 10
01-DEC-03 03.57.30 PM 01-DEC-03 03.57.39 PM 000A000A00000029 U 11
01-DEC-03 03.57.39 PM 01-DEC-03 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';

3)Flashback Transaction Query
A Flashback Transaction Query is a query on the view 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 & shows the complete transaction steps for that xid ...eg

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

4)Flashback Table
Just like the flashback query helps retrieve rows of a table, FLASHBACK TABLE helps restore the state of a table to a certain point in time even if a table structure changed has occurred since then. The following simple command will take us to the table state at the specified timestamp.
eg
SQL> FLASHBACK TABLE Employee TO
TIMESTAMP ('13-SEP-04 8:50:58','DD-MON-YY HH24: MI: SS');


5)Flashback Drop (Recycle Bin)
Dropping of objects by accident has always been a problem for users and DBAs alike. Users soon realize their mistake but then it's too late and historically there is 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.

What is the Recycle Bin?
The Recycle Bin is a virtual container where all dropped objects reside. Underneath the covers, the objects are occupying 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 dependant objects are not moved, they are simply renamed with a prefix of BIN$. You can continue to access the data in a dropped table
or even use Flashback Query against it.

SQL>FLASHBACK TABLE dept_test TO BEFORE DROP;


6)Flashback Database
We have talked abt object recovery berore this , now lets talk what if something happens to database , this 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

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

Eg :
-- 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


7)Flashback Query Functions
The TIMESTAMP_TO_SCN and SCN_TO_TIMESTAMP functions have been added to SQL and PL/SQL to simplify flashback operations:
eg :
DECLARE
l_scn NUMBER;
l_timestamp TIMESTAMP;
BEGIN
l_scn := TIMESTAMP_TO_SCN(SYSTIMESTAMP - 1/24);
l_timestamp := SCN_TO_TIMESTAMP(l_scn);
END;
/

Limits

• There is one little catch you need to be aware of: it doesn't work on the system tablespaces.
• As fas as the performance overhead of enabling Flashback Database is concerned its less than 2%.

Conclusion:
Flashback is an insurance feature. FLASHBACK should be considered another tool in the belt, rather than a luxury that allows us to be careless about the data simply because we have the ability to recover it easily.

Used References and Recommended Readings

1. Oracle Database Backup and Recovery Basics - 10g Release 2 (10.2)
2. Oracle Database Backup and Recovery Advanced User's Guide - 10g Release 2 (10.2)
3. Oracle Database Backup and Recovery Reference - 10g Release 2 (10.2)