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

This statement
sets the transaction isolation level for the next transaction, globally, or
for the current session.

The default behavior of SET TRANSACTION is to set the isolation level
for the next (not yet
started) transaction.  If you use the GLOBAL keyword, the statement
sets the default transaction level globally for all new connections
created from that point on. Existing connections are unaffected.
You need the SUPER
privilege to do this.  Using the SESSION keyword sets the
default transaction level for all future transactions performed on the
current connection.

For descriptions of each InnoDB transaction isolation level, see
[InnoDB transaction isolation, InnoDB transaction isolation].
InnoDB supports each of these levels
from MySQL 4.0.5 on. The default level is REPEATABLE READ.

You can set the initial default global isolation level for mysqld with
the --transaction-isolation option.
See also : [Server options].
