PL/SQL SET TRANSACTION

In Oracle PL/SQL, transactions play a crucial role in ensuring data integrity and consistency. The SET TRANSACTION statement allows developers to configure transaction properties like isolation levels and read/write modes. Understanding this statement is essential for managing concurrency and performance effectively.

Syntax

The basic syntax of SET TRANSACTION is:

SET TRANSACTION
  [ READ { ONLY | WRITE } ]
  [ ISOLATION LEVEL { SERIALIZABLE | READ COMMITTED } ]
  [ USE ROLLBACK SEGMENT rollback_segment_name ]
  [ NAME 'transaction_name' ];

Explanation of Options:

1. READ ONLY / WRITE

READ ONLY: The transaction can only execute SELECT statements and cannot modify data.

WRITE: Allows both SELECT and data modification (INSERT, UPDATE, DELETE).

2. ISOLATION LEVEL

READ COMMITTED (default): A query sees only committed data and does not lock rows it reads.

SERIALIZABLE: Ensures transaction isolation by preventing other transactions from modifying the data being read.

3. USE ROLLBACK SEGMENT

Specifies a rollback segment for the transaction, used in manual undo management (rarely needed in modern Oracle versions).

4. NAME

Assigns a name to the transaction for tracking and debugging.

Examples

1. Setting a Read-Only Transaction

SET TRANSACTION READ ONLY;
SELECT * FROM employees WHERE department_id = 10;
-- Any attempt to INSERT, UPDATE, or DELETE will fail.

2. Setting a Serializable Transaction

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
UPDATE accounts SET balance = balance - 500 WHERE account_id = 101;

This prevents other transactions from modifying the same data until the transaction is committed or rolled back.

3. Naming a Transaction

SET TRANSACTION NAME 'OrderProcessing';
INSERT INTO orders (order_id, customer_id, amount) VALUES (1001, 500, 250.00);

This helps in debugging by tracking the transaction name in database logs.

Best Practices

Use READ ONLY for reports to avoid unnecessary row locks.

Use SERIALIZABLE only when strict consistency is needed, as it can lead to contention.

Avoid unnecessary SET TRANSACTION statements if default settings suffice.

Conclusion

The SET TRANSACTION statement in Oracle PL/SQL is a powerful tool for managing transaction behavior. Proper use of isolation levels and read/write modes helps optimize performance while maintaining data integrity. Understanding when and how to use this statement can significantly improve database efficiency.