In this MySQL manual page: http://dev.mysql.com/doc/refman/5.6/en/implicit-commit.html I read: "UNLOCK TABLES commits a transaction only if any tables currently have been locked with LOCK TABLES to acquire nontransactional table locks." However, this doesn't seem to be the case, at least with MariaDB 10.0: MariaDB [test]> CREATE OR REPLACE TABLE m (c INT) ENGINE = InnoDB; Query OK, 0 rows affected (0.39 sec) MariaDB [test]> CREATE OR REPLACE TABLE t (c INT UNIQUE) ENGINE = InnoDB; Query OK, 0 rows affected (0.54 sec) MariaDB [test]> SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; Query OK, 0 rows affected (0.00 sec) MariaDB [test]> START TRANSACTION; Query OK, 0 rows affected (0.00 sec) MariaDB [test]> INSERT INTO m VALUES (1); Query OK, 1 row affected (0.00 sec) MariaDB [test]> INSERT INTO t VALUES (1); Query OK, 1 row affected (0.00 sec) MariaDB [test]> UNLOCK TABLES; Query OK, 0 rows affected (0.00 sec) MariaDB [test]> SELECT @@in_transaction; +------------------+ | @@in_transaction | +------------------+ | 1 | +------------------+ 1 row in set (0.00 sec) -- So, could you please confirm that UNLOCK TABLES is transaction-safe? If it isn't, could someone please explain in which cases it isn't? Regards Federico