Oracle Flashback Technology

8/8/2019 Oracle Flashback Technology

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Oracle Flashback Technology

Flashback Recovery is a new enhancement to the 10g database for the DBA'stoolkit. Effectively, it's an "Oh shit!" protection mechanism for DBAs as itreduces recovery time from hours to minutes. Ask any DBA about the main

cause of application outage - other than hardware failure - and the answer willbe "human error". Such errors can result in logical data corruption and canbring down the complete system. Part of being human is making mistakes.Without advance planning, these errors are extremely difficult to avoid, andcan be very difficult to recover from. Typical user-errors may include theaccidental deletion of valuable data, deleting the wrong data, or dropping thewrong 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, asit revolutionizes recovery by working just on the changed data.Flashback provides:

An effective way to recover from complex human errorsFaster database point-in-time recoverySimplified management and administrationLittle performance overheadThe performance overhead of enabling Flashback Database is less than 2%.While you may not be willing to sacrifice any performance overhead for yourproduction database, think about the trade-off. If you could recover thedatabase in minutes instead of hours, saving your company millions of dollars inlost revenue, wouldn't you be willing to give 2% of the resources to FlashbackDatabase?Below is a chat to show how quickly Flashback can recover your database.


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Architecture:There are some basic prerequisites for Flashback recovery:The database should be in Archive Log Mode. Issue the alter databasearchivelog 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 deleteddata to be maintained in the database. E.g. alter system setDB_FLASHBACK_RETENTION_TARGET=4320, i.e. 72 hoursDB_RECOVERY_FILE_DEST_SIZE: This specifies the maximum data that can beretained. E.g. alter system set DB_RECOVERY_FILE_DEST_SIZE=536870912, i.e.512 MBDB_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 recoveriesThere are six basic types of Flashback recovery, discussed below in detail:

1. Flashback Query

2. Flashback Version Query3. Flashback Transaction Query4. Flashback Table5. Flashback Drop (Recycle Bin)6. Flashback Database

Flashback Query:You can perform a Flashback Query using a SELECT statement with an AS OFclause. You can use a Flashback Query to retrieve data as it existed at sometime in the past. The query explicitly references a past time using a timestampor 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 aspecified time in the past. Suppose, for instance, that a DBA discovers at 12:30PM that data for employee JON has been deleted from the employee table, andthe DBA knows that at 9:30AM the data for JON was correctly stored in thedatabase. The DBA can use a Flashback Query to examine the contents of thetable at 9:30, to find out what data has been lost. If appropriate, the DBA canthen re-insert the lost data in the database.The following query retrieves the state of the employee record for JOHN at9:30AM, 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 TIMESTAMPTO_TIMESTAMP('2006-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS')WHERE name = 'JON');


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Flashback Query FunctionsThe TIMESTAMP_TO_SCN and SCN_TO_TIMESTAMP functions have been added toSQL and PL/SQL to simplify flashback operations:DECLARE

l_scn NUMBER;

l_timestamp TIMESTAMP;BEGINl_scn := TIMESTAMP_TO_SCN(SYSTIMESTAMP - 1/24);l_timestamp := SCN_TO_TIMESTAMP(l_scn);

END;/

Flashback Version QueryThe Flashback Version Query is a powerful tool for the DBA to run analysis andanswer 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 forauditing 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 rowVERSIONS_OPERATION - The action that created this version of the row (suchas delete, insert, and update)VERSIONS_STARTSCN - The SCN in which this row version first occurredVERSIONS_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 minvalueand 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 1001-DEC-06 03.57.30 PM 01-DEC-06 03.57.39 PM 000A000A00000029 U 1101-DEC-06 03.57.39 PM 01-DEC-06 03.57.55 PM 000A000B00000029 U 12We can automatically generate the undo statement using:

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

Flashback Transaction Query


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Auditing Table Changes Using Flashback Transaction Query

A Flashback Transaction Query is a query on a view calledFLASHBACK_TRANSACTION_QUERY. You use a Flashback Transaction Query to

obtain transaction information, including SQL code that you can use to undoeach of the changes made by the transaction. It uses the XID from above andshows 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 FROMflashback_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

Recovering Tables Using the Flashback Table Feature

Just as the flashback query helps to retrieve rows of a table, FLASHBACK TABLEhelps to restore the state of a table to a certain point in time - even if a tablestructure change has occurred since then. The following simple command willtake 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 andDBAs alike. Users soon realize their mistake, but by then it's too late - and untilnow there has been no easy way to recover those dropped tables, indexes,constraints, triggers, etc.


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Flashback Drop provides a safety net when dropping objects in Oracle Database10g. When a user drops a table, Oracle automatically places it into the RecycleBin.The Recycle BinThe 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 wascreated in the USERS tablespace, the dropped table EMP remains in theUSERS tablespace. Dropped tables and any associated objects such as indexes,constraints, nested tables, and other dependent objects are not moved; theyare simply renamed with the prefix BIN$$. You can continue to access the datain a dropped table, or even use Flashback Query on it.SQL> FLASHBACK TABLE dept_test TO BEFORE DROP;

Flashback DatabaseWe have been talking about object recovery. But what if something happens tothe whole database? This recovery quickly rewinds the complete database to a

previous time, to correct any logical data corruption. This can be used withboth 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_scnWhen the system comes back with FLASHBACK COMPLETE, open the databasewith Resetlogs.For example:-- Create a dummy table.

CREATE TABLE flashback_database_test (id NUMBER(10)

--Flashback 5 MinutesCONN sys/password AS SYSDBASHUTDOWN IMMEDIATESTARTUP MOUNT EXCLUSIVEFLASHBACK DATABASE TO TIMESTAMP SYSDATE-(1/24/12);ALTER DATABASE OPEN RESETLOGS;

-- Check that the table is gone.DESC flashback_database_test

Oracle Flashback Technology

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