57 DBMS_FLASHBACK

Using DBMS_FLASHBACK, you can flash back to a version of the database at a specified wall-clock time or a specified system change number (SCN).

This chapter contains the following topics:


Using DBMS_FLASHBACK


Overview

DBMS_FLASHBACK provides an interface for the user to view the database at a particular time in the past, with the additional capacity provided by transaction backout features that allow for selective removal of the effects of individual transactions. This should not be confused this with a flashback database which actually moves the database back in time.

When DBMS_FLASHBACK is enabled, the user session uses the Flashback version of the database, and applications can execute against the Flashback version of the database.

You may want to use DBMS_FLASHBACK for the following reasons:

  • Self-service repair: If you accidentally delete rows from a table, you can recover the deleted rows.

  • Packaged applications such as e-mail and voicemail: You can use Flashback to restore deleted e-mail by re-inserting the deleted message into the current message box.

  • Decision support system (DSS) and online analytical processing (OLAP) applications: You can perform data analysis or data modeling to track seasonal demand.


Security Model

To use this package, a database administrator must grant EXECUTE privileges for DBMS_FLASHBACK.


Types

The following types are used by DBMS_FLASHBACK subprograms:

Table 57-1 DBMS_FLASHBACK

Type Description

TXNAME_ARRAY

Creates a VARRAY for holding Transaction Names or Identifiers (XIDs



Exceptions

Table 57-2 DBMS_FLASHBACK Error Messages

Error Description

ORA-08180

Time specified is too old.

ORA-08181

Invalid system change number specified.

ORA-08182

User cannot begin read-only or serializable transactions in Flashback mode.

ORA-08183

User cannot enable Flashback within an uncommitted transaction.

ORA-08184

User cannot enable Flashback within another Flashback session.

ORA-08185

SYS cannot enable Flashback mode.



Operational Notes

DBMS_FLASHBACK is automatically turned off when the session ends, either by disconnection or by starting another connection.

PL/SQL cursors opened in Flashback mode return rows as of the flashback time or SCN. Different concurrent sessions (connections) in the database can perform Flashback to different wall-clock times or SCNs. DML and DDL operations and distributed operations are not allowed while a session is running in Flashback mode. You can use PL/SQL cursors opened before disabling Flashback to perform DML.

Under Automatic Undo Management (AUM) mode, you can use retention control to control how far back in time to go for the version of the database you need.If you need to perform a Flashback over a 24-hour period, the DBA should set the undo_retention parameter to 24 hours. This way, the system retains enough undo information to regenerate the older versions of the data.

You can set the RETENTION GUARANTEE clause for the undo tablespace to ensure that unexpired undo is not discarded.UNDO_RETENTION is not in itself a complete guarantee because, if the system is under space pressure, unexpired undo may be overwritten with freshly generated undo. In such cases, RETENTION GUARANTEE prevents this. For more information, see the Oracle Database Administrator's Guide

In a Flashback-enabled session, SYSDATE will not be affected; it will continue to provide the current time.

DBMS_FLASHBACK can be used within logon triggers to enable Flashback without changing the application code.


Examples

The following example illustrates how Flashback can be used when the deletion of a senior employee triggers the deletion of all the personnel reporting to him. Using the Flashback feature, you can recover and re-insert the missing employees.

DROP TABLE employee;
DROP TABLE keep_scn;

REM -- Keep_scn is a temporary table to store scns that we are interested in

CREATE TABLE keep_scn (scn number); 
SET ECHO ON 
CREATE TABLE employee ( 
   employee_no   number(5) PRIMARY KEY, 
   employee_name varchar2(20), 
   employee_mgr  number(5) 
      CONSTRAINT mgr_fkey REFERENCES EMPLOYEE ON DELETE CASCADE, 
   salary        number, 
   hiredate      date 
); 

REM -- Populate the company with employees
INSERT INTO employee VALUES (1, 'John Doe', null, 1000000, '5-jul-81'); 
INSERT INTO employee VALUES (10, 'Joe Johnson', 1, 500000, '12-aug-84'); 
INSERT INTO employee VALUES (20, 'Susie Tiger', 10, 250000, '13-dec-90'); 
INSERT INTO employee VALUES (100, 'Scott Tiger', 20, 200000, '3-feb-86'); 
INSERT INTO employee VALUES (200, 'Charles Smith', 100, 150000, '22-mar-88'); 
INSERT INTO employee VALUES (210, 'Jane Johnson', 100, 100000, '11-apr-87'); 
INSERT INTO employee VALUES (220, 'Nancy Doe', 100, 100000, '18-sep-93'); 
INSERT INTO employee VALUES (300, 'Gary Smith', 210, 75000, '4-nov-96'); 
INSERT INTO employee VALUES (310, 'Bob Smith', 210, 65000, '3-may-95'); 
COMMIT; 

REM -- Show the entire org
SELECT lpad(' ', 2*(level-1)) || employee_name Name 
FROM employee 
CONNECT BY PRIOR employee_no = employee_mgr 
START WITH employee_no = 1 
ORDER BY LEVEL; 

REM -- Sleep for a short time (approximately 10 to 20  seconds) to avoid 
REM -- querying close to table creation

EXECUTE DBMS_LOCK.SLEEP(10);

REM -- Store this snapshot for later access through Flashback
DECLARE 
I NUMBER; 
BEGIN 
I := DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER; 
INSERT INTO keep_scn VALUES (I); 
COMMIT; 
END;
/

REM -- Scott decides to retire but the transaction is done incorrectly
DELETE FROM EMPLOYEE WHERE employee_name = 'Scott Tiger'; 
COMMIT; 

REM -- notice that all of scott's employees are gone 
SELECT lpad(' ', 2*(level-1)) || employee_name Name 
FROM EMPLOYEE 
CONNECT BY PRIOR employee_no = employee_mgr 
START WITH employee_no = 1 
ORDER BY LEVEL; 

REM -- Flashback to see Scott's organization
DECLARE 
   restore_scn number; 
BEGIN 
   SELECT  scn INTO restore_scn FROM keep_scn; 
   DBMS_FLASHBACK.ENABLE_AT_SYSTEM_CHANGE_NUMBER (restore_scn); 
END; 
/ 

REM -- Show Scott's org.
SELECT lpad(' ', 2*(level-1)) || employee_name Name 
FROM employee 
CONNECT BY PRIOR employee_no = employee_mgr 
START WITH employee_no = 
   (SELECT employee_no FROM employee WHERE employee_name = 'Scott Tiger') 
ORDER BY LEVEL; 

REM -- Restore scott's organization.
DECLARE 
   scotts_emp NUMBER; 
   scotts_mgr NUMBER; 
   CURSOR c1 IS 
      SELECT employee_no, employee_name, employee_mgr, salary, hiredate 
      FROM employee 
      CONNECT BY PRIOR employee_no = employee_mgr 
      START WITH employee_no = 
         (SELECT employee_no FROM employee WHERE employee_name = 'Scott Tiger'); 
   c1_rec c1 % ROWTYPE; 
BEGIN 
   SELECT employee_no, employee_mgr INTO scotts_emp, scotts_mgr FROM employee 
   WHERE employee_name = 'Scott Tiger'; 
   /* Open c1 in flashback mode */
   OPEN c1; 
   /* Disable Flashback */
   DBMS_FLASHBACK.DISABLE; 
 LOOP 
   FETCH c1 INTO c1_rec; 
   EXIT WHEN c1%NOTFOUND; 
   /*
     Note that all the DML operations inside the loop are performed
     with Flashback disabled
   */
   IF (c1_rec.employee_mgr = scotts_emp) then 
      INSERT INTO employee VALUES (c1_rec.employee_no, 
         c1_rec.employee_name, 
         scotts_mgr, 
         c1_rec.salary, 
         c1_rec.hiredate); 
   ELSE 
   IF (c1_rec.employee_no != scotts_emp) THEN 
   INSERT INTO employee VALUES (c1_rec.employee_no, 
         c1_rec.employee_name, 
         c1_rec.employee_mgr, 
         c1_rec.salary, 
         c1_rec.hiredate); 
      END IF; 
    END IF; 
 END LOOP; 
END; 
/ 

REM -- Show the restored organization.
select lpad(' ', 2*(level-1)) || employee_name Name 
FROM employee 
CONNECT BY PRIOR employee_no = employee_mgr 
START WITH employee_no = 1 
ORDER BY LEVEL; 

Summary of DBMS_FLASHBACK Subprograms

Table 57-3 DBMS_FLASHBACK Package Subprograms

Subprogram Description

DISABLE Procedure

Disables the Flashback mode for the entire session

ENABLE_AT_SYSTEM_CHANGE_NUMBER Procedure

Enables Flashback for the entire session. Takes an SCN as an Oracle number and sets the session snapshot to the specified number. Inside the Flashback mode, all queries will return data consistent as of the specified wall-clock time or SCN

ENABLE_AT_TIME Procedure

Enables Flashback for the entire session. The snapshot time is set to the SCN that most closely matches the time specified in query_time

GET_SYSTEM_CHANGE_NUMBER Function

Returns the current SCN as an Oracle number. You can use the SCN to store specific snapshots

TRANSACTION_BACKOUT Procedures

Provides the mechanism to backout a transaction



DISABLE Procedure

This procedure disables the Flashback mode for the entire session.

Syntax

DBMS_FLASHBACK.DISABLE;

Examples

The following example queries the salary of an employee, Joe, on August 30, 2000:

EXECUTE dbms_flashback.enable_at_time('30-AUG-2000');
SELECT salary FROM emp where name = 'Joe'
EXECUTE dbms_flashback.disable;

ENABLE_AT_SYSTEM_CHANGE_NUMBER Procedure

This procedure takes an SCN as an input parameter and sets the session snapshot to the specified number. In the Flashback mode, all queries return data consistent as of the specified wall-clock time or SCN. It enables Flashback for the entire session.

Syntax

DBMS_FLASHBACK.ENABLE_AT_SYSTEM_CHANGE_NUMBER (
   query_scn IN NUMBER);

Parameters

Table 57-4 ENABLE_AT_SYSTEM_CHANGE_NUMBER Procedure Parameters

Parameter Description

query_scn

The system change number (SCN), a version number for the database that is incremented on every transaction commit.



ENABLE_AT_TIME Procedure

This procedure enables Flashback for the entire session. The snapshot time is set to the SCN that most closely matches the time specified in query_time.It enables Flashback for the entire session.

Syntax

DBMS_FLASHBACK.ENABLE_AT_TIME (
   query_time   IN TIMESTAMP);

Parameters

Table 57-5 ENABLE_AT_TIME Procedure Parameters

Parameter Description

query_time

This is an input parameter of type TIMESTAMP. A time stamp can be specified in the following ways:

  • Using the TIMESTAMP constructor

    EXECUTE DBMS_FLASHBACK.ENABLE_AT_TIME(TIMESTAMP '2001-01-09 12:31:00'). 
    

    Use the Globalization Support (NLS) format and supply a string. The format depends on the Globalization Support settings.

  • Using the TO_TIMESTAMP function:

    EXECUTE DBMS_FLASHBACK.ENABLE_AT_TIME(TO_TIMESTAMP('12-02-2001 14:35:00', 'DD-MM-YYYY HH24:MI:SS'))
    

    You provide the format you want to use. This example shows the TO_TIMESTAMP function for February 12, 2001, 2:35 PM.

  • If the time is omitted from query time, it defaults to the beginning of the day, that is, 12:00 A.M.

  • Note that if the query time contains a time zone, the time zone information is truncated.



GET_SYSTEM_CHANGE_NUMBER Function

This function returns the current SCN as an Oracle number datatype. You can obtain the current change number and store it for later use. This helps you retain specific snapshots.

Syntax

DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER
 RETURN NUMBER;

TRANSACTION_BACKOUT Procedures

This procedure provides a mechanism to backout a set of transactions. The user can call these procedures with either transaction names or transaction identifiers (XIDS).

The procedure analyzes the transactional dependencies, perform DML's and generates an extensive report on the operation performed by the subprogram. This procedure does not commit the DML's performed as part of transaction backout. However it will hold all the required locks on rows and tables in the right form, so that no other dependencies can enter the system. To make the changes permanent the user needs to explicitly commit the transaction.

A report will be generated in the system tables DBA_FLASHBACK_TRANSACTION_STATE and DBA_FLASHBACK_TRANSACTION_REPORT.

Syntax

DBMS_FLASHBACK.TRANSACTION_BACKOUT
   numberofxids       NUMBER,
   xids               XID_ARRAY,
   options            NUMBER default NOCASCADE, 
   timeHint           TIMESTAMP default MINTIME);

DBMS_FLASHBACK.TRANSACTION_BACKOUT
   numberofxids       NUMBER,  
   xids               XID_ARRAY, 
   options            NUMBER default NOCASCADE,
   scnHint            TIMESTAMP default 0   );

DBMS_FLASHBACK.TRANSACTION_BACKOUT
   numberofxids      NUMBER,
   txnnames          TXNAME_ARRAY,
   options           NUMBER default NOCASCADE,
   timehint          TIMESTAMP MINTIME );

DBMS_FLASHBACK.TRANSACTION_BACKOUT
   numberofxids     NUMBER, 
   txnNames         TXNAME_ARRAY, 
   options          NUMBER default NOCASCADE,
   scnHint          NUMBER 0);

Parameters

Table 57-6 TRANSACTION_BACKOUT Procedure Parameters

Parameter Description

numberofxids

Number of transactions passed as input

xids

List of transaction IDs in the form of an array

txnnames

List of transaction names in the form of an array

options

Backout dependent transactions:

  • NOCASCADE - The user expects no dependency. If a dependency is found, this raises an error, with the first dependent transaction provided in the report.

  • NOCASCADE_FORCE - The user forcibly backs out the given transactions without considering the dependent transactions. The RDBMS executes the UNDO SQL for the given transactions in reverse order of their commit times. If no constraints break, and the result is satisfactory, the user can either COMMIT the changes or else ROLL BACK.

  • NONCONFLICT_ONLY - This option lets the user backout the changes to the non-conflicting rows of the given transactions. Note that a transaction dependency will happen due to a row conflict, whether either through WAW or primary/unique key constraints. If the user chooses to back out only the non-conflicting rows, this should not cause any problem with database consistency, although transaction atomicity is lost. As this is a recovery operation, the user can correct the data.

  • CASCADE - This will completely remove the given transactions including their dependents in a post order fashion (reverse order of commit times).

timehint

Time hint on the start of the transaction

scnhint

SCN hint on the start of the transaction


Usage Notes

  • If transaction name is used, a time hint must be provided. The time hint should be a time before the start of all the given transactions to back out.

  • If the SCN hint is provided, it must be before the start of the earliest transaction in the specified input set, or this will raise an error and terminate. If it is not provided and the transaction has committed within undo retention, the database system is able to determine the start time.