13.3.7 SET TRANSACTION Syntax

To set the transaction isolation level, use an ISOLATION LEVEL level clause. It is not permitted to specify multiple ISOLATION LEVEL clauses in the same SET TRANSACTION statement.

The default isolation level is REPEATABLE READ. Other permitted values are READ COMMITTED, READ UNCOMMITTED, and SERIALIZABLE. For information about these isolation levels, see Section 15.7.2.1, “Transaction Isolation Levels”.

To set the transaction access mode, use a READ WRITE or READ ONLY clause. It is not permitted to specify multiple access-mode clauses in the same SET TRANSACTION statement.

By default, a transaction takes place in read/write mode, with both reads and writes permitted to tables used in the transaction. This mode may be specified explicitly using SET TRANSACTION with an access mode of READ WRITE.

If the transaction access mode is set to READ ONLY, changes to tables are prohibited. This may enable storage engines to make performance improvements that are possible when writes are not permitted.

In read-only mode, it remains possible to change tables created with the TEMPORARY keyword using DML statements. Changes made with DDL statements are not permitted, just as with permanent tables.

The READ WRITE and READ ONLY access modes also may be specified for an individual transaction using the START TRANSACTION statement.

You can set transaction characteristics globally, for the current session, or for the next transaction only:

A change to global transaction characteristics requires the CONNECTION_ADMIN or SUPER privilege. Any session is free to change its session characteristics (even in the middle of a transaction), or the characteristics for its next transaction (prior to the start of that transaction).

To set the global 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 permissible values are READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE.

Similarly, to set the global transaction access mode at server startup, use the --transaction-read-only option. The default is OFF (read/write mode) but the value can be set to ON for a mode of read only.

For example, to set the isolation level to REPEATABLE READ and the access mode to READ WRITE, use these lines in the [mysqld] section of an option file:

[mysqld]
transaction-isolation = REPEATABLE-READ
transaction-read-only = OFF

At runtime, characteristics at the global, session, and next-transaction scope levels can be set indirectly using the SET TRANSACTION statement, as described previously. They can also be set directly using the SET statement to assign values to the transaction_isolation and transaction_read_only system variables:

The following tables show the characteristic scope level set by each SET TRANSACTION and variable-assignment syntax.

It is possible to check the global and session values of transaction characteristics at runtime: