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
This statement sets the transaction isolation level or the transaction access mode globally, for the current session, or for the next transaction:
- With the
GLOBALkeyword, the statement sets the default transaction level globally for all subsequent sessions. Existing sessions are unaffected.
- With the
SESSIONkeyword, the statement sets the default transaction level for all subsequent transactions performed within the current session.
- Without any
GLOBALkeyword, 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.
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
SERIALIZABLE. For example, to set the default isolation
REPEATABLE READ, use these lines in the
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,
The following sections describe how MariaDB supports the different transaction levels.
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
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 (
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
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.
This is the default isolation level for InnoDB. For consistent reads,
there is an important difference from the
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
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 minimum isolation level for non-distributed XA transactions.
This level is like REPEATABLE READ, but InnoDB implicitly converts all
plain SELECT statements to
SELECT ... LOCK IN SHARE MODE if
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.
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.
SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
Attempting to set the isolation level within an existing transaction without specifying
START TRANSACTION; SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; ERROR 1568 (25001): Transaction characteristics can't be changed while a transaction is in progress