How To View and Change the Current Transaction Isolation Level in MySQL?

If you want to view or change the current transaction isolation level, you can use the following commands:

• SELECT @@TX_ISOLATION FROM DUAL; -- Viewing the current transaction isolation level.
• SET TRANSACTION ISOLATION LEVEL levelName; -- Changing the current transaction isolation level.

The tutorial exercise below shows you how to view and change transaction isolation level:

>\mysql\bin\mysql -u dev -piyf fyi

mysql> SELECT @@TX_ISOLATION FROM DUAL;
+-----------------+
| @@TX_ISOLATION  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.46 sec)

mysql> SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
Query OK, 0 rows affected (0.10 sec)

mysql> SELECT @@TX_ISOLATION FROM DUAL;
+----------------+
| @@TX_ISOLATION |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set (0.02 sec)

How To Test Transaction Isolation Levels?

If you want to test transaction isolation levels, you need to make sure that:

• The tables are created with transaction-safe storage engines, like InnoDB.
• Multiple-statement transactions are started with "START TRANSACTION" commands.
• "SET TRANSACTION ISOLATION LEVEL" commands are used to set the right isolation level.

The tutorial exercise below shows you a good example of testing the REPEATABLE-READ transaction isolation level:

>\mysql\bin\mysql -u dev -piyf fyi

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM fyi_links;
+-----+---------------+-------+--------+--------------------
| id  | url           | notes | counts | created           
+-----+---------------+-------+--------+--------------------
| 101 | fyicenter.com | Good  |    999 | 2006-07-01 20:34:10
| 110 | centerfyi.com | Wrong |      0 | 2006-07-01 20:34:12
| 112 | oracle.com    | NULL  |   NULL | 2006-07-01 20:41:12
| 113 | mysql.com     | NULL  |   NULL | 2006-07-01 20:41:21
+-----+---------------+-------+--------+--------------------
4 rows in set (0.00 sec)

Keep the "dev" mysql window as is, and open another window to run another instance of mysql:

>\mysql\bin\mysql -u dev -piyf fyi

mysql> DELETE FROM fyi_links WHERE id = 112;
Query OK, 1 row affected (0.09 sec)

mysql> DELETE FROM fyi_links WHERE id = 113;
Query OK, 1 row affected (0.25 sec)

Go back to the first "dev" mysql window.

mysql> SELECT * FROM fyi_links;
+-----+---------------+-------+--------+--------------------
| id  | url           | notes | counts | created           
+-----+---------------+-------+--------+--------------------
| 101 | fyicenter.com | Good  |    999 | 2006-07-01 20:34:10
| 110 | centerfyi.com | Wrong |      0 | 2006-07-01 20:34:12
| 112 | oracle.com    | NULL  |   NULL | 2006-07-01 20:41:12
| 113 | mysql.com     | NULL  |   NULL | 2006-07-01 20:41:21
+-----+---------------+-------+--------+--------------------
4 rows in set (0.00 sec)

mysql> COMMIT;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM fyi_links;
+-----+---------------+-------+--------+--------------------
| id  | url           | notes | counts | created           
+-----+---------------+-------+--------+--------------------
| 101 | fyicenter.com | Good  |    999 | 2006-07-01 20:34:10
| 110 | centerfyi.com | Wrong |      0 | 2006-07-01 20:34:12
+-----+---------------+-------+--------+--------------------
2 rows in set (0.00 sec)

As you can see that two records were deleted from the second session between two executions of the same SELECT statement in the first session. The deleted records were still showing up in the output until the transaction was ended with the COMMIT statement.

⇒ What Is a Data Lock in MySQL

⇐ Transaction Isolation Levels in MySQL

⇑ Transaction Management: Commit or Rollback in MySQL

⇑⇑ MySQL Database Tutorials