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13.3.1 START TRANSACTION, COMMIT, and ROLLBACK Syntax

Inhoudsopgave
  1. START TRANSACTION
  2.     [transaction_characteristic [, transaction_characteristic] ...]
  3.  
  4. transaction_characteristic: {
  5.     WITH CONSISTENT SNAPSHOT
  6.   | READ WRITE
  7.   | READ ONLY
  8. }
  9.  
  10. BEGIN [WORK]
  11. COMMIT [WORK] [AND [NO] CHAIN] [[NO] RELEASE]
  12. ROLLBACK [WORK] [AND [NO] CHAIN] [[NO] RELEASE]
  13. SET autocommit = {0 | 1}

These statements provide control over use of transactions:

  • START TRANSACTION or BEGIN start a new transaction.

  • COMMIT commits the current transaction, making its changes permanent.

  • ROLLBACK rolls back the current transaction, canceling its changes.

  • SET autocommit disables or enables the default autocommit mode for the current session.

By default, MySQL runs with autocommit mode enabled. This means that, when not otherwise inside a transaction, each statement is atomic, as if it were surrounded by START TRANSACTION and COMMIT. You cannot use ROLLBACK to undo the effect; however, if an error occurs during statement execution, the statement is rolled back.

To disable autocommit mode implicitly for a single series of statements, use the START TRANSACTION statement:

  1. START TRANSACTION;
  2. SELECT @A:=SUM(salary) FROM table1 WHERE type=1;
  3. UPDATE table2 SET summary=@A WHERE type=1;
  4. COMMIT;

With START TRANSACTION, autocommit remains disabled until you end the transaction with COMMIT or ROLLBACK. The autocommit mode then reverts to its previous state.

START TRANSACTION permits several modifiers that control transaction characteristics. To specify multiple modifiers, separate them by commas.

  • The WITH CONSISTENT SNAPSHOT modifier 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 15.7.2.3, “Consistent Nonlocking Reads”. The WITH CONSISTENT SNAPSHOT modifier does not change the current transaction isolation level, so it provides a consistent snapshot only if the current isolation level is one that permits a consistent read. The only isolation level that permits a consistent read is REPEATABLE READ. For all other isolation levels, the WITH CONSISTENT SNAPSHOT clause is ignored. A warning is generated when the WITH CONSISTENT SNAPSHOT clause is ignored.

  • The READ WRITE and READ ONLY modifiers set the transaction access mode. They permit or prohibit changes to tables used in the transaction. The READ ONLY restriction prevents the transaction from modifying or locking both transactional and nontransactional tables that are visible to other transactions; the transaction can still modify or lock temporary tables.

    MySQL enables extra optimizations for queries on InnoDB tables when the transaction is known to be read-only. Specifying READ ONLY ensures these optimizations are applied in cases where the read-only status cannot be determined automatically. See Section 8.5.3, “Optimizing InnoDB Read-Only Transactions” for more information.

    If no access mode is specified, the default mode applies. Unless the default has been changed, it is read/write. It is not permitted to specify both READ WRITE and READ ONLY in the same statement.

    In read-only mode, it remains possible to change tables created with the TEMPORARY keyword using DML statements. Changes made with DDL statements are not permitted, just as with permanent tables.

    For additional information about transaction access mode, including ways to change the default mode, see Section 13.3.7, “SET TRANSACTION Syntax”.

    If the read_only system variable is enabled, explicitly starting a transaction with START TRANSACTION READ WRITE requires the CONNECTION_ADMIN or SUPER privilege.

Important

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 28, Connectors and APIs, or the documentation for your API, for more information.

To disable autocommit mode explicitly, use the following statement:

  1. SET autocommit=0;

After disabling autocommit mode by setting the autocommit variable to zero, changes to transaction-safe tables (such as those for InnoDB or NDB) are not made permanent immediately. You must use COMMIT to store your changes to disk or ROLLBACK to ignore the changes.

autocommit is a session variable and must be set for each session. To disable autocommit mode for each new connection, see the description of the autocommit system variable at Section 5.1.8, “Server System Variables”.

BEGIN and BEGIN WORK are supported as aliases of START TRANSACTION for initiating a transaction. START TRANSACTION is standard SQL syntax, is the recommended way to start an ad-hoc transaction, and permits modifiers that BEGIN does not.

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 13.6.1, “BEGIN ... END Compound-Statement Syntax”.

Note

Within all stored programs (stored procedures and functions, triggers, and events), the parser treats BEGIN [WORK] as the beginning of a BEGIN ... END block. Begin a transaction in this context with START TRANSACTION instead.

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 5.1.8, “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 new transaction also uses the same access mode (READ WRITE or READ ONLY) 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.

Beginning a transaction causes any pending transaction to be committed. See Section 13.3.3, “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 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 nontransactional table within a transaction, an ER_WARNING_NOT_COMPLETE_ROLLBACK warning occurs. Changes to transaction-safe tables are rolled back, but not changes to nontransaction-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 nontransactional tables cannot be rolled back. If a transaction that is rolled back includes modifications to nontransactional tables, the entire transaction is logged with a ROLLBACK statement at the end to ensure that modifications to the nontransactional tables are replicated.) See Section 5.4.4, “The Binary Log”.

You can change the isolation level or access mode for transactions with the SET TRANSACTION statement. See Section 13.3.7, “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.

Note

In MySQL 8.0, BEGIN, COMMIT, and ROLLBACK are not affected by --replicate-do-db or --replicate-ignore-db rules.


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