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Oracle 9i New Features

--Sridevi Edupuganti

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Oracle9i FlashbackQuery

Scope & Application

Package and Procedures Used

Prerequisites List of steps

Oracle9i Flashback Example

Performance considerations Limitations of Flashback query

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SCOPE &

APPLICATION

This is intended for anyone willing to useFlashback for any user error recovery.

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Package and procedures

used

Package

DBMS_FLASHBACK

Procedures disable

enable_at_time

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Prerequisites

Set up the database for Flashback Use

Prerequisite : Set init.ora parameter

UNDO_MANAGEMENT = AUTO

UNDO_TABLESPACE =UNDOTBS

UNDO_RETENTION =1200

The ALTER SYSTEM SET undo_retention = 1200 urges Oracle to keep

undo information for 1200 seconds.

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undo_retention

ALTER SYSTEM SET undo_retention = 1200

This command sets 1200 seconds during which

undo information is retained. All committed undoinformation in the system is retained for at least

20 minutes(20*60=1200)

This ensures that all queries running for 20minutes or less do not receive an ORA-1555

(snapshot too old) error, under normal

circumstances.

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List of steps

step1: a) Create a FLASH user

b) grant the required privileges

step2: a) Create a user table and insert rowsb) Create another table to keep

the time when rows existed

step 3: a) Execute disable procedure

b) Delete a row

step 4: Enable Flashback

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Flashback Example

Step 1

Set up User

Create a flash userSQL> create user flash identified by gordon;

Grant required privileges

SQL>grant connect, resource to flash;SQL>grant execute on dbms_flashback to flash;

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Flashback Example

Step 2 Setup Table

Create a user table

CREATE TABLE tst(t number(5)); Insert values into the above table

INSERT INTO tst VALUES(1);INSERT INTO tst VALUES(2);COMMIT;

Create another table to keep the time when rows existed

CREATE TABLE keep_date (date_scn_tracking date);

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Flashback Example

SQL> SELECT * FROM tst;

T

----------1

2

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Flashback Example

Using disable procedure

SQL> execute dbms_flashback.disable;SQL>INSERT INTO keep_date select sysdate from dual;

SQL> COMMIT;

SQL>DELETE FROM tst WHERE t = 1;

SQL>COMMIT;

SQL> SELECT * FROM tst;

T----------

2

Step 3

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Flashback Example

Using enable procedure

SQL> declarerestore_scn date;

beginselect date_scn_tracking into restore_scn

from keep_date;dbms_flashback.enable_at_time(restore_scn);

end;

Step 4

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Result after using enable

procedure

SQL> SELECT * FROM tst;

T

----------12

Flashback is able to display the 2 rows that existed in

the past, before the row deletion.

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Performance

considerations

When a query is executed for a time in the past, asnapshot of data as it was at that time needs to berecreated using "undo" data. For every piece of data

that has changed since the time of the FlashbackQuery, the corresponding undo data needs to beretrieved and compiled. Hence the performance ofthe Flashback Query degrades significantly as thevolume of data which needs to "recreated" increases.

f

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Performance

considerations

Because of this, a Flashback Query works bestwhen it is used to select a small set of data,preferably using indexes. If a Flashback Query

must perform a full table scan, its performancewill depend on the amount of DML activitiesperformed on that table between the presentand the time of the data that the FlashbackQuery is retrieving.

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Limitations of Flashbackquery:

1. Specifying a time will only find the flashback

copy to the nearest five minute interval. This is

also true of the get_system_change_number.

2. You can never flashback more than 5 days,

irrespective of UNDO_RETENTION.

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