Don’t Assume – Transactions

MySQL by default is a NON transactional database. For the hobbyist (See The Hobbyist and the Professional ), startup entrepreneur and website developer this may not appear foreign, however to the seasoned Oracle DBA who has only used Oracle the concept is very foreign.

In MySQL you have to be concerned with two situations that will catch the unprepared out. The first is the default autocommit mode. This is TRUE, i.e. all statements are automatically committed on completion.

mysql> SELECT @@autocommit,TRUE;
+--------------+------+
| @@autocommit | TRUE |
+--------------+------+
|            1 |    1 |
+--------------+------+
1 row in set (0.00 sec)

The second is the storage engine used. Again a foreign term for Oracle DBA’s, a storage engine is a technology that stores and retrieves the underlying data from the MySQL database. MySQL has many different storage engines, each with relative strengths and weaknesses and different features. For the purpose of this discussion it is important to know that engines are either non-transactional or transactional. The default storage engine MyISAM is NON transactional. MySQL provides by default the InnoDB storage engine which is transactional. There are distinct advantages of a non transactional environment which I will not go into at this time.

Having recently written about this in my upcoiming book Expert PHP and MySQL I will demonstrate what happens with both MyISAM and InnoDB.

Non-transactional Tables

To show the difference, Listing 6-7 demonstrates that atomicity is not possible with non-transactional tables. The following tables are used in this example.

DROP TABLE IF EXISTS non_trans_parent;
CREATE TABLE non_trans_parent (
  id   INT UNSIGNED NOT NULL AUTO_INCREMENT,
  val  VARCHAR(10) NOT NULL,
PRIMARY KEY (id),
UNIQUE KEY (val)
) ENGINE=MyISAM DEFAULT CHARSET latin1;
DROP TABLE IF EXISTS non_trans_child;
CREATE TABLE non_trans_child (
  id        INT UNSIGNED NOT NULL AUTO_INCREMENT,
  parent_id INT UNSIGNED NOT NULL,
  created   TIMESTAMP NOT NULL,
PRIMARY KEY (id),
INDEX (parent_id)
) ENGINE=MyISAM DEFAULT CHARSET latin1;

To test things out, perform a sample transaction that inserts records into these two tables:

START TRANSACTION;
INSERT INTO non_trans_parent(val) VALUES(‘a’);
INSERT INTO non_trans_child(parent_id,created) VALUES(LAST_INSERT_ID(),NOW());
INSERT INTO non_trans_parent (val) VALUES(‘a’);
ERROR 1062 (23000): Duplicate entry ‘a’ for key ‘val’
ROLLBACK;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> SHOW WARNINGS;
+--------+------+--------------------------------------------------------------
| Level  | Code | Message
+--------+------+--------------------------------------------------------------
| Warning| 1196 | Some non-transactional changed tables couldn’t be rolled back
+--------+------+--------------------------------------------------------------
SELECT * FROM non_trans_parent;
+----+-----+
| id | val |
+----+-----+
|  1 | a   |
+----+-----+
SELECT * FROM non_trans_child;
+----+-----------+---------------------+
| id | parent_id | created             |
+----+-----------+---------------------+
|  1 |         1 | 2009–09–21 23:44:25 |
+----+-----------+---------------------+

As you can see, data that you would have expected to not exist from the transaction is present.

Transactional Tables

Repeat these SQL statements using the transactional storage engine InnoDB; you will observe the difference between transactional and non transactional processing. The following tables, shown in Listing 6-8, are used in this example.

DROP TABLE IF EXISTS trans_parent;
CREATE TABLE trans_parent (
  id   INT UNSIGNED NOT NULL AUTO_INCREMENT,
  val  VARCHAR(10) NOT NULL,
PRIMARY KEY (id),
UNIQUE KEY (val)
) ENGINE=InnoDB DEFAULT CHARSET latin1;
DROP TABLE IF EXISTS trans_child;
CREATE TABLE trans_child (
  id        INT UNSIGNED NOT NULL AUTO_INCREMENT,
  parent_id INT UNSIGNED NOT NULL,
  created   TIMESTAMP NOT NULL,
PRIMARY KEY (id),
INDEX (parent_id)
) ENGINE=InnoDB DEFAULT CHARSET latin1;

Perform a sample transaction that inserts records into these two tables:

START TRANSACTION;
INSERT INTO trans_parent (val) VALUES(‘a’);
INSERT INTO trans_child (parent_id,created) VALUES(LAST_INSERT_ID(),NOW());
INSERT INTO trans_parent (val) VALUES(‘a’);
ERROR 1062 (23000): Duplicate entry ‘a’ for key ‘val’
ROLLBACK;
Query OK, 0 rows affected (0.01 sec)
SELECT * FROM trans_parent;
Empty set (0.00 sec)
SELECT * FROM trans_child;
Empty set (0.00 sec)

As you can see, no data has been recorded as part of the failing transaction.

About “Don’t Assume”

“Don’t Assume” is a series of posts to help the Oracle DBA understand, use and appreciate the subtle differences and unique characteristics of the MySQL RDBMS in comparison to Oracle. These points as essential to operate MySQL effectively in a production environment and avoid any loss of data or availability.

For more posts in this series be sure to follow the mysql4oracledba tag and also watch out for MySQL for Oracle DBA presentations.

The MySQLCamp for the Oracle DBA is a series of educational talks all Oracle DBA resources should attend. Two presentations from this series IGNITION and LIFTOFF will be presented at the MySQL Users Conference 2010 in Santa Clara, April 2010 This series also includes JUMPSTART and VELOCITY . If you would like to here these presentations in your area, please contact me .