Friday, April 17, 2009


SET autocommit = {0 | 1}
The START TRANSACTION or BEGIN statement begins a new transaction. COMMIT commits the current transaction, making its changes permanent. ROLLBACK rolls back the current transaction, canceling its changes. The SET autocommit statement disables or enables the default autocommit mode for the current session.
Beginning with MySQL 5.0.3, the optional WORK keyword is supported for COMMIT and ROLLBACK, as are the CHAIN and RELEASE clauses. CHAIN and RELEASE can be used for additional control over transaction completion. The value of the completion_type system variable determines the default completion behavior. See Section 5.1.3, “Server System Variables”.
The AND CHAIN clause causes a new transaction to begin as soon as the current one ends, and the new transaction has the same isolation level as the just-terminated transaction. The RELEASE clause causes the server to disconnect the current client session after terminating the current transaction. Including the NO keyword suppresses CHAIN or RELEASE completion, which can be useful if the completion_type system variable is set to cause chaining or release completion by default.
By default, MySQL runs with autocommit mode enabled. This means that as soon as you execute a statement that updates (modifies) a table, MySQL stores the update on disk to make it permanent. To disable autocommit mode, use the following statement:
SET autocommit=0;
After disabling autocommit mode by setting the autocommit variable to zero, changes to transaction-safe tables (such as those for InnoDB, BDB, or NDBCLUSTER) are not made permanent immediately. You must use COMMIT to store your changes to disk or ROLLBACK to ignore the changes.
To disable autocommit mode for a single series of statements, use the START TRANSACTION statement:
SELECT @A:=SUM(salary) FROM table1 WHERE type=1;
UPDATE table2 SET summary=@A WHERE type=1;
With START TRANSACTION, autocommit remains disabled until you end the transaction with COMMIT or ROLLBACK. The autocommit mode then reverts to its previous state.
BEGIN and BEGIN WORK are supported as aliases of START TRANSACTION for initiating a transaction. START TRANSACTION is standard SQL syntax and is the recommended way to start an ad-hoc transaction.
Many APIs used for writing MySQL client applications (such as JDBC) provide their own methods for starting transactions that can (and sometimes should) be used instead of sending a START TRANSACTION statement from the client. See Chapter 20, Connectors and APIs, or the documentation for your API, for more information.
The BEGIN statement differs from the use of the BEGIN keyword that starts a BEGIN ... END compound statement. The latter does not begin a transaction. See Section 12.8.1, “BEGIN ... END Compound Statement Syntax”.
You can also begin a transaction like this:
The WITH CONSISTENT SNAPSHOT clause starts a consistent read for storage engines that are capable of it. This applies only to InnoDB. The effect is the same as issuing a START TRANSACTION followed by a SELECT from any InnoDB table. See Section, “Consistent Non-Locking Reads”. The WITH CONSISTENT SNAPSHOT clause does not change the current transaction isolation level, so it provides a consistent snapshot only if the current isolation level is one that allows consistent read (REPEATABLE READ or SERIALIZABLE).
Beginning a transaction causes any pending transaction to be committed. See Section 12.4.3, “Statements That Cause an Implicit Commit”, for more information.
Beginning a transaction also causes table locks acquired with LOCK TABLES to be released, as though you had executed UNLOCK TABLES. Beginning a transaction does not release a global read lock acquired with FLUSH TABLES WITH READ LOCK.
For best results, transactions should be performed using only tables managed by a single transaction-safe storage engine. Otherwise, the following problems can occur:
If you use tables from more than one transaction-safe storage engine (such as InnoDB and BDB), and the transaction isolation level is not SERIALIZABLE, it is possible that when one transaction commits, another ongoing transaction that uses the same tables will see only some of the changes made by the first transaction. That is, the atomicity of transactions is not guaranteed with mixed engines and inconsistencies can result. (If mixed-engine transactions are infrequent, you can use SET TRANSACTION ISOLATION LEVEL to set the isolation level to SERIALIZABLE on a per-transaction basis as necessary.)
If you use tables that are not transaction-safe within a transaction, changes to those tables are stored at once, regardless of the status of autocommit mode.
If you issue a ROLLBACK statement after updating a non-transactional table within a transaction, an ER_WARNING_NOT_COMPLETE_ROLLBACK warning occurs. Changes to transaction-safe tables are rolled back, but not changes to non-transaction-safe tables.
Each transaction is stored in the binary log in one chunk, upon COMMIT. Transactions that are rolled back are not logged. (Exception: Modifications to non-transactional tables cannot be rolled back. If a transaction that is rolled back includes modifications to non-transactional tables, the entire transaction is logged with a ROLLBACK statement at the end to ensure that modifications to the non-transactional tables are replicated.) See Section 5.2.3, “The Binary Log”.
You can change the isolation level for transactions with SET TRANSACTION ISOLATION LEVEL. See Section 12.4.6, “SET TRANSACTION Syntax”.
Rolling back can be a slow operation that may occur implicitly without the user having explicitly asked for it (for example, when an error occurs). Because of this, SHOW PROCESSLIST displays Rolling back in the State column for the session, not only for explicit rollbacks performed with the ROLLBACK statement but also for implicit rollbacks.
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