9i falsback query
Post on 10-Feb-2018
230 Views
Preview:
TRANSCRIPT
-
7/22/2019 9i Falsback Query
1/17
1
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
Oracle 9i New Features
--Sridevi Edupuganti
-
7/22/2019 9i Falsback Query
2/17
2
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
Oracle9i FlashbackQuery
Scope & Application
Package and Procedures Used
Prerequisites List of steps
Oracle9i Flashback Example
Performance considerations Limitations of Flashback query
-
7/22/2019 9i Falsback Query
3/17
3
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
SCOPE &
APPLICATION
This is intended for anyone willing to useFlashback for any user error recovery.
-
7/22/2019 9i Falsback Query
4/17
4
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
Package and procedures
used
Package
DBMS_FLASHBACK
Procedures disable
enable_at_time
-
7/22/2019 9i Falsback Query
5/17
5
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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.
-
7/22/2019 9i Falsback Query
6/17
6
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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.
-
7/22/2019 9i Falsback Query
7/177
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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
-
7/22/2019 9i Falsback Query
8/178
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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;
-
7/22/2019 9i Falsback Query
9/179
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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);
-
7/22/2019 9i Falsback Query
10/1710
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
Flashback Example
SQL> SELECT * FROM tst;
T
----------1
2
-
7/22/2019 9i Falsback Query
11/1711
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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
-
7/22/2019 9i Falsback Query
12/1712
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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
-
7/22/2019 9i Falsback Query
13/1713
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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.
-
7/22/2019 9i Falsback Query
14/1714
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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
-
7/22/2019 9i Falsback Query
15/1715
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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.
-
7/22/2019 9i Falsback Query
16/1716
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
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.
-
7/22/2019 9i Falsback Query
17/1717
..our people make the dif ference.. I nstituti onalizing Excell ence
Main Menu
top related