Autonomous Transaction

AUTONOMOUS_TRANSACTION Pragma: The AUTONOMOUS_TRANSACTION pragma changes the way a subprogram works within a transaction. A subprogram marked with this pragma can do SQL operations and commit or roll back those operations, without committing or rolling back the data in the main transaction. Autonomous Transactions provide a new method of controlling transactions in stored procedures. Autonomous Transactions allow you to create a new subtransaction that may commit or rollback changes independent of the parent transaction.

Syntax pragma autonomous_transaction ::=

PRAGMA Signifies that the statement is a pragma (compiler directive). Pragmas are processed at compile time, not at run time. They pass information to the compiler. Usage Notes : You can apply this pragma to: • • • •

Top-level (not nested) anonymous PL/SQL blocks Local, standalone, and packaged functions and procedures Methods of a SQL object type Database triggers

You cannot apply this pragma to an entire package or an entire an object type. Instead, you can apply the pragma to each packaged subprogram or object method. You can code the pragma anywhere in the declarative section. For readability, code the pragma at the top of the section. Once started, an autonomous transaction is fully independent. It shares no locks, resources, or commit-dependencies with the main transaction. You can log events, increment retry counters, and so on, even if the main transaction rolls back. Unlike regular triggers, autonomous triggers can contain transaction control statements such as COMMIT and ROLLBACK, and can issue DDL statements (such as CREATE and DROP) through the EXECUTE IMMEDIATE statement.

Changes made by an autonomous transaction become visible to other transactions when the autonomous transaction commits. The changes also become visible to the main transaction when it resumes, but only if its isolation level is set to READ COMMITTED (the default). If you set the isolation level of the main transaction to SERIALIZABLE, changes made by its autonomous transactions are not visible to the main transaction when it resumes. In the main transaction, rolling back to a savepoint located before the call to the autonomous subprogram does not roll back the autonomous transaction. Remember, autonomous transactions are fully independent of the main transaction. If an autonomous transaction attempts to access a resource held by the main transaction (which cannot resume until the autonomous routine exits), a deadlock can occur. Oracle raises an exception in the autonomous transaction, which is rolled back if the exception goes unhandled. If you try to exit an active autonomous transaction without committing or rolling back, Oracle raises an exception. If the exception goes unhandled, or if the transaction ends because of some other unhandled exception, the transaction is rolled back.

Pros and Cons of Autonomous Transactions: There are not many drawbacks that come to mind with autonomous transactions. Once you understand how they work and what they are useful for, they work without any ‘gotchas’. They have very few limitations or restrictions and, as they have been implemented for a long time in Oracle as recursive SQL, they are very well tested. Pros

Cons

Allows you to commit in a trigger

Cannot perform parallel queries in autonomous transactions. These queries will execute serially.

Allows you to perform DML from a SELECT

Deadlocks may occur more frequently as a single user can now deadlock themselves

Allows for more modular code with less side effects (avoids the "hey – you rolled back my work!")

Must be used in a top level anonymous block, procedure, or function. Cannot be included in a nested PL/SQL block.

Allows you to implement auditing that cannot be rolled back

As it is an extension of recursive SQL, it is a feature that has been around internally for a long time (tested).

EXAMPLES: 1) Declaring an Autonomous Function in a Package CREATE OR REPLACE PACKAGE emp_actions AS -- package specification FUNCTION raise_salary (emp_id NUMBER, sal_raise NUMBER) RETURN NUMBER; END emp_actions; / CREATE OR REPLACE PACKAGE BODY emp_actions AS -- package body -- code for function raise_salary FUNCTION raise_salary (emp_id NUMBER, sal_raise NUMBER) RETURN NUMBER IS PRAGMA AUTONOMOUS_TRANSACTION; new_sal NUMBER(8,2); BEGIN UPDATE employees SET salary = salary + sal_raise WHERE employee_id = emp_id; COMMIT; SELECT salary INTO new_sal FROM employees WHERE employee_id = emp_id; RETURN new_sal; END raise_salary; END emp_actions; / 2) Declaring an Autonomous Standalone Procedure CREATE PROCEDURE lower_salary (emp_id NUMBER, amount NUMBER) AS PRAGMA AUTONOMOUS_TRANSACTION; BEGIN UPDATE employees SET salary = salary - amount WHERE employee_id = emp_id; COMMIT; END lower_salary; /

3) Declaring an Autonomous PL/SQL Block DECLARE PRAGMA AUTONOMOUS_TRANSACTION; emp_id NUMBER(6); amount NUMBER(6,2); BEGIN emp_id := 200; amount := 200; UPDATE employees SET salary = salary - amount WHERE employee_id = emp_id; COMMIT; END; /