MySQL 5.5 Reference Manual

12.3.6. `SET TRANSACTION` Syntax
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12.3.6. `SET TRANSACTION` Syntax

SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL
  {
       READ UNCOMMITTED
     | READ COMMITTED
     | REPEATABLE READ
     | SERIALIZABLE
   }

This statement sets the transaction isolation level 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.

SET TRANSACTION ISOLATION LEVEL without GLOBAL or SESSION is not permitted while there is an active transaction:

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.02 sec)

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

To set the global default isolation level at server startup, use the --transaction-isolation=level option to mysqld on the command line or in an option file. Values of level for this option use dashes rather than spaces, so the permissible 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 transaction 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 Section 13.6.9.4, “InnoDB Record, Gap, and Next-Key Locks”, and Section 13.6.9.6, “Locks Set by Different SQL Statements in InnoDB”.

The following list describes how MySQL supports the different transaction levels:

READ UNCOMMITTED
SELECT statements are performed in a nonlocking 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 (nonlocking) reads: Each consistent read, even within the same transaction, sets and reads its own fresh snapshot. See Section 13.6.9.2, “Consistent Nonlocking Reads”.
For locking reads (SELECT with FOR UPDATE or LOCK IN SHARE MODE), InnoDB locks only index records, not the gaps before them, and thus permits 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
In MySQL 5.5, 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 nonmatching rows are released after MySQL has evaluated the WHERE condition.
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 (nonlocking) SELECT statements within the same transaction, these SELECT statements are consistent also with respect to each other. See Section 13.6.9.2, “Consistent Nonlocking Reads”.
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.
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 (nonlocking) 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.)

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12.3.6. `SET TRANSACTION` Syntax