autocommit下，每条语句组成一个事务：
"In InnoDB, all user activity occurs inside a transaction. If autocommit mode is enabled, each SQL statement forms a single transaction on its own"
"By default, MySQL starts the session for each new connection with autocommit enabled, so MySQL does a commit after each SQL statement if that statement did not return an error."
http://dev.mysql.com/doc/refman/5.5/en/innodb-transaction-model.html