When Commit Transaction Fails: Is Automatic Rollback Guaranteed?

Automatic Rollback without COMMIT TRANSACTION: Is It Safe?

The question arises about the automatic rollback of a transaction when COMMIT TRANSACTION is not executed due to syntax errors. In this scenario, a transaction with multiple statements is initiated with START TRANSACTION, but encountering a syntax error causes the transaction to fail.

Immediate Rollback Assumption

The user assumes that the transaction is rolled back immediately upon encountering an error, as the COMMIT TRANSACTION statement is never reached. However, this assumption is not entirely accurate.

Rollback Policy

In general, transactions are not rolled back immediately when an error occurs. However, certain conditions can trigger an automatic rollback:

Client Application Policy

Some client applications employ a policy that automatically rolls back transactions upon encountering errors. For instance, the mysql command-line client typically exits upon errors, causing an implicit rollback.

Exceptions

Apart from client application policy, two exceptions can cause automatic rollback:

• Quitting (disconnecting): Terminating the connection always rolls back any active transaction.
• Lock-wait timeouts or deadlocks: These conditions can implicitly trigger a rollback.

Manual Control

In most cases, however, when an error is encountered within a transaction, the error is returned to the application, leaving it up to the developer to determine whether or not to commit the transaction.

Conclusion

While it may seem that transactions are automatically rolled back when an error occurs, this is not universally true. It depends on the client application's policy and specific exceptions that can trigger an automatic rollback. Developers should always consider the implications of potential errors when executing transactions and make appropriate error handling decisions to ensure data integrity.

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