Syntax

SET [GLOBAL | SESSION] TRANSACTION
    transaction_property [, transaction_property] ...

transaction_property:
    ISOLATION LEVEL level
  | READ WRITE
  | READ ONLY

level:
     REPEATABLE READ
   | READ COMMITTED
   | READ UNCOMMITTED
   | SERIALIZABLE

Description

This statement sets the transaction isolation level or the transaction access mode globally, for the current session, or for the next transaction:

  • With the GLOBAL keyword, the statement sets the default transaction level globally for all subsequent sessions. Existing sessions are unaffected.
  • With the SESSION keyword, the statement sets the default transaction level for all subsequent transactions performed within the current session.
  • Without any SESSION or GLOBAL keyword, the statement sets the isolation level for the next (not started) transaction performed within the current session.

A change to the global default isolation level requires the SUPER privilege. Any session is free to change its session isolation level (even in the middle of a transaction), or the isolation level for its next transaction.

Isolation level

To set the global default isolation level at server startup, use the --transaction-isolation=level option on the command line or in an option file. Values of level for this option use dashes rather than spaces, so the allowable values are READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. For example, to set the default isolation level to REPEATABLE READ, use these lines in the [mysqld] section of an option file:

[mysqld] transaction-isolation = REPEATABLE-READ

To determine the global and session transaction isolation levels at runtime, check the value of the tx_isolation system variable:

SELECT @@GLOBAL.tx_isolation, @@tx_isolation;

InnoDB supports each of the translation isolation levels described here using different locking strategies. The default level is REPEATABLE READ. For additional information about InnoDB record-level locks and how it uses them to execute various types of statements, see XtraDB/InnoDB Lock Modes, and http://dev.mysql.com/doc/refman/en/innodb-locks-set.html.

Isolation Levels

The following sections describe how MariaDB supports the different transaction levels.

READ UNCOMMITTED

SELECT statements are performed in a non-locking fashion, but a possible earlier version of a row might be used. Thus, using this isolation level, such reads are not consistent. This is also called a "dirty read." Otherwise, this isolation level works like READ COMMITTED.

READ COMMITTED

A somewhat Oracle-like isolation level with respect to consistent (non-locking) reads: Each consistent read, even within the same transaction, sets and reads its own fresh snapshot. See http://dev.mysql.com/doc/refman/en/innodb-consistent-read.html.

For locking reads (SELECT with FOR UPDATE or LOCK IN SHARE MODE), InnoDB locks only index records, not the gaps before them, and thus allows the free insertion of new records next to locked records. For UPDATE and DELETE statements, locking depends on whether the statement uses a unique index with a unique search condition (such as WHERE id = 100), or a range-type search condition (such as WHERE id > 100). For a unique index with a unique search condition, InnoDB locks only the index record found, not the gap before it. For range-type searches, InnoDB locks the index range scanned, using gap locks or next-key (gap plus index-record) locks to block insertions by other sessions into the gaps covered by the range. This is necessary because "phantom rows" must be blocked for MySQL replication and recovery to work.

Note: Since MariaDB 5.1, if the READ COMMITTED isolation level is used or the innodb_locks_unsafe_for_binlog system variable is enabled, there is no InnoDB gap locking except for foreign-key constraint checking and duplicate-key checking. Also, record locks for non-matching rows are released after MariaDB has evaluated the WHERE condition. As of MariaDB/MySQL 5.1, if you use READ COMMITTED or enable innodb_locks_unsafe_for_binlog, you must use row-based binary logging.

REPEATABLE READ

This is the default isolation level for InnoDB. For consistent reads, there is an important difference from the READ COMMITTED isolation level: All consistent reads within the same transaction read the snapshot established by the first read. This convention means that if you issue several plain (non-locking) SELECT statements within the same transaction, these SELECT statements are consistent also with respect to each other. See http://dev.mysql.com/doc/refman/en/innodb-consistent-read.html.

For locking reads (SELECT with FOR UPDATE or LOCK IN SHARE MODE), UPDATE, and DELETE statements, locking depends on whether the statement uses a unique index with a unique search condition, or a range-type search condition. For a unique index with a unique search condition, InnoDB locks only the index record found, not the gap before it. For other search conditions, InnoDB locks the index range scanned, using gap locks or next-key (gap plus index-record) locks to block insertions by other sessions into the gaps covered by the range.

This is the only transaction level that can be used with the row based binary logging.

This is the minimum isolation level for non-distributed XA transactions.

SERIALIZABLE

This level is like REPEATABLE READ, but InnoDB implicitly converts all plain SELECT statements to SELECT ... LOCK IN SHARE MODE if autocommit is disabled. If autocommit is enabled, the SELECT is its own transaction. It therefore is known to be read only and can be serialized if performed as a consistent (non-locking) read and need not block for other transactions. (This means that to force a plain SELECT to block if other transactions have modified the selected rows, you should disable autocommit.)

Distributed XA transactions should always use this isolation level.

Access mode

These clauses appeared in MariaDB 10.0.

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. The only exception to this rule is that read only transactions can perform DDL statements on temporary tables.

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 START TRANSACTION statement, in which case the specified mode is only valid for one transaction.

Examples

SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Attempting to set the isolation level within an existing transaction without specifying GLOBAL or SESSION.

START TRANSACTION;

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
ERROR 1568 (25001): Transaction characteristics can't be changed while a transaction is in progress

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