Read Consistency

Read Consistency :

• Read consistency guarantees a consistent view of the data at all times.
• Changes made by one user do not conflict with changes made by another user.
• Read consistency ensures that on the same data :
• Readers do not wait for writers
• Writers do not wait for readers

Implementation of Read Consistency :

State of the Data Before COMMIT or ROLLBACK

• The previous state of the data can be recovered.
• The current user can review the results of the DML operations by using the SELECT statement.
• Other users cannot view the results of the DML statements by the current user.
• The affected rows are locked; other users cannotchange the data in the affected rows.

State of the Data After COMMIT :

• Data changes are made permanent in the database.
• The previous state of the data is permanently lost.
• All users can view the results.
• Locks on the affected rows are released; those rows are available for other users to
  manipulate.
• All savepoints are erased.

Committing Data :

Make the changes :

• BEGIN
  
• DELETE FROM emp WHERE empno = 7090;
• INSERT INTO dept VALUES (290, 'Corporate Tax', NULL);

Commit the changes :

• COMMIT;

State of the Data After ROLLBACK :
Discard all pending changes by using the ROLLBACK statement :

• Data changes are undone.
• Previous state of the data is restored.
• Locks on the affected rows are released.

• BEGIN;
• DELETE FROM copy_emp;
• DELETE 20
• ROLLBACK ;
• Rollback