EmpID EmpName Salary

1000 Tom 2500

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

Story of Transaction

• ABC company is maintaining the above table for its employees.

EmpID EmpName Salary

1000 Tom 2500

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

• Salary values of employees are updated now and then in this table.

•Here 2 rows are going to be updated.

Story of Transaction

3000

5000

EmpID EmpName Salary

1000 Tom 3000

1001 Jerry 3000

1002 Mickey 5000

1003 Frank 1200

• We have the updated table with us.

Story of Transaction

• One day, the system got crashed.

• No Backup!! And what they have is just the previous table .

Story of Transaction

EmpID EmpName Salary

1000 Tom 2500

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

• From that day on wards, they planned to track or log each changes they are making on the table. These log files could help them in these situations

• They understood that, there are two techniques to maintain log files

Story of Transaction

Technique 1:

Story of Transaction

EmpID EmpName Salary

1000 Tom 2500

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

Table in database

Technique 1:

Story of Transaction

EmpID EmpName Salary

1000 Tom 2500

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

BEGIN

UPDATE tbl_emp SET Salary=3000 WHERE EmpID=1000;

DBMS_OUTPUT.PUT_LINE(‘Third row’);

UPDATE tbl_emp SET Salary=5000 WHERE EmpID=1002;

COMMIT;

END;

PL/SQL code Transaction Log File

<T1 Starts>

<T1, Row1, 3000>

<T1, Row3, 5000>

<T1, Commit>

EmpID EmpName Salary

1000 Tom 3000

1001 Jerry 3000

1002 Mickey 5000

1003 Frank 1200

Deferred update

Technique 2:

Story of Transaction

EmpID EmpName Salary

1000 Tom 2500

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

BEGIN

UPDATE tbl_emp SET Salary=3000 WHERE EmpID=1000;

DBMS_OUTPUT.PUT_LINE(‘Third row’);

UPDATE tbl_emp SET Salary=5000 WHERE EmpID=1002;

COMMIT;

END;

PL/SQL code Transaction Log File

<T1 Starts>

<T1, Row1, 2500, 3000>

<T1, Row3, 4500, 5000>

<T1, Commit>

EmpID EmpName Salary

1000 Tom 3000

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

EmpID EmpName Salary

1000 Tom 3000

1001 Jerry 3000

1002 Mickey 5000

1003 Frank 1200

Immediate update

• ABC company now has the updated table and log file with them. Log files are frequently saved to safe place as its small & quick.

Story of Transaction

• Now, If the system crashes again, let us see how ABC recovers data from log file.

Story of Transaction

EmpID EmpName Salary

1000 Tom 2500

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

<T1 Starts>

<T1, Row1, 3000>

<T1, Row3, 5000>

<T1, Commit>

Have taken Deferred Update

Transaction Log file

EmpID EmpName Salary

1000 Tom 3000

1001 Jerry 3000

1002 Mickey 4500

1003 Frank 1200

EmpID EmpName Salary

1000 Tom 3000

1001 Jerry 3000

1002 Mickey 5000

1003 Frank 1200

Both Start and Commit is there. So it was a successful

transaction

EmpID EmpName Salary

1000 Tom 3000

1001 Jerry 3000

1002 Mickey 5000

1003 Frank 1200