The Ultimate Guide to High Availability with MariaDB

START TRANSACTION

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Syntax

START TRANSACTION [transaction_property [, transaction_property] ...] | BEGIN [WORK]
COMMIT [WORK] [AND [NO] CHAIN] [[NO] RELEASE]
ROLLBACK [WORK] [AND [NO] CHAIN] [[NO] RELEASE]
SET autocommit = {0 | 1}

transaction_property:
    WITH CONSISTENT SNAPSHOT
  | READ WRITE
  | READ ONLY

Contents

  1. Syntax
  2. Description
    1. Access Mode
    2. autocommit
    3. DDL Statements
    4. in_transaction
    5. WITH CONSISTENT SNAPSHOT
  3. Examples
  4. See Also

Description

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.

START TRANSACTION and SET autocommit = 1 implicitly commit the current transaction, if any.

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.

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.

Access Mode

The access mode specifies whether the transaction is allowed to write data or not. By default, transactions are in READ WRITE mode (see the tx_read_only system variable). READ ONLY mode allows the storage engine to apply optimizations that cannot be used for transactions which write data. Note that unlike the global read_only mode, READ_ONLY ADMIN (and SUPER before MariaDB 10.11.0) privilege doesn't allow writes and DDL statements on temporary tables are not allowed either.

It is not permitted to specify both READ WRITE and READ ONLY in the same statement.

READ WRITE and READ ONLY can also be specified in the SET TRANSACTION statement, in which case the specified mode is valid for all sessions, or for all subsequent transaction used by the current session.

autocommit

By default, MariaDB runs with autocommit mode enabled. This means that as soon as you execute a statement that updates (modifies) a table, MariaDB 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 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.

DDL Statements

DDL statements (CREATE, ALTER, DROP) and administrative statements (FLUSH, RESET, OPTIMIZE, ANALYZE, CHECK, REPAIR, CACHE INDEX), transaction management statements (BEGIN, START TRANSACTION) and LOAD DATA INFILE, cause an implicit COMMIT and start a new transaction. An exception to this rule are the DDL that operate on temporary tables: you can CREATE, ALTER and DROP them without causing any COMMIT, but those actions cannot be rolled back. This means that if you call ROLLBACK, the temporary tables you created in the transaction will remain, while the rest of the transaction will be rolled back.

Transactions cannot be used in Stored Functions or Triggers. In Stored Procedures and Events BEGIN is not allowed, so you should use START TRANSACTION instead.

A transaction acquires a metadata lock on every table it accesses to prevent other connections from altering their structure. The lock is released at the end of the transaction. This happens even with non-transactional storage engines (like MEMORY or CONNECT), so it makes sense to use transactions with non-transactional tables.

in_transaction

The in_transaction system variable is a session-only, read-only variable that returns 1 inside a transaction, and 0 if not in a transaction.

WITH CONSISTENT SNAPSHOT

The WITH CONSISTENT SNAPSHOT option starts a consistent read for storage engines such as InnoDB that can do so, the same as if a START TRANSACTION followed by a SELECT from any InnoDB table was issued.

See Enhancements for START TRANSACTION WITH CONSISTENT SNAPSHOT.

Examples

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

See Also

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