bitsCN.com

1、innodb_rollback_on_timeout变量

下面是MySQL官方手册关开innodb_rollback_on_timeout变量的说明：

In MySQL 5.0.13 and up, InnoDB rolls back only the last statement on a transaction timeout by default. If --innodb_rollback_on_timeout is specified, a transaction timeout causes InnoDB to abort and roll back the entire transaction (the same behavior as before MySQL 5.0.13). This variable was added in MySQL 5.0.32.

该变量默认值为OFF，如果事务因为加锁超时，会回滚上一条语句执行的操作。如果设置ON，则整个事务都会回滚。

下面通过一个示例来验证上面这段话。

2、示例

(1) innodb_rollback_on_timeout为OFF

Session 1

Session 2

mysql> create table tt(c1 int primary key, c2 int)engine=innodb;

Query OK, 0 rows affected (0.01 sec)

mysql> insert into tt values(1, 1);

Query OK, 1 row affected (0.00 sec)

mysql> begin;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from tt where c1=1 lock in share mode;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

+----+------+

1 row in set (0.00 sec)

mysql> begin;

Query OK, 0 rows affected (0.00 sec)

mysql> insert into tt values(10,10);

Query OK, 1 row affected (0.00 sec)

mysql> delete from tt where c1=1;

ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

mysql> select * from tt;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

| 10 | 10 |

+----+------+

2 rows in set (0.00 sec)

mysql> rollback;

Query OK, 0 rows affected (0.01 sec)

mysql> select * from tt;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

+----+------+

1 row in set (0.00 sec)

mysql> select * from tt;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

+----+------+

1 row in set (0.00 sec)

mysql> begin;

Query OK, 0 rows affected (0.00 sec)

mysql> insert into tt values(10,10);

Query OK, 1 row affected (0.00 sec)

mysql> delete from tt where c1=1;

ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

mysql> commit;

Query OK, 0 rows affected (0.02 sec)

mysql> select * from tt;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

| 10 | 10 |

+----+------+

2 rows in set (0.00 sec)

session2因为加锁超时，事务回退到上一条语句。

(2) innodb_rollback_on_timeout为ON

Session 1

Session 2

mysql> begin;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from tt where c1=1 lock in share mode;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

+----+------+

1 row in set (0.00 sec)

mysql> begin;

Query OK, 0 rows affected (0.00 sec)

mysql> insert into tt values(11,11);

Query OK, 1 row affected (0.00 sec)

mysql> delete from tt where c1=1;

ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

mysql> select * from tt;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

| 10 | 10 |

+----+------+

2 rows in set (0.00 sec)

mysql> commit;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from tt;

+----+------+

| c1 | c2 |

+----+------+

| 1 | 1 |

| 10 | 10 |

+----+------+

2 rows in set (0.00 sec)

session2加锁超时，整个事务回滚。

3、总结

innodb_rollback_on_timeout为OFF，事务会回滚到上一个保存点，InnoDB在执行每条SQL语句之前，都会创建一个保存点，参见代码：

int

row_insert_for_mysql(

/* out: error code or DB_SUCCESS */

byte* mysql_rec, /* in: row in the MySQL format */

row_prebuilt_t* prebuilt) /* in: prebuilt struct in MySQL

handle */

{

。。。

savept = trx_savept_take(trx);

。。。

如果事务因为加锁超时，相当于回滚到上一条语句。但是报错后，事务还没有完成，用户可以选择是继续提交，或者回滚之前的操作，由用户选择是否进一步提交或者回滚事务。

innodb_rollback_on_timeout为ON，整个事务都会回滚。这可以从row_mysql_handle_errors函数中得到验证。

ibool
row_mysql_handle_errors(
/*====================*/
                /* out: TRUE if it was a lock wait and
                we should continue running the query thread */
    ulint*        new_err,/* out: possible new error encountered in
                lock wait, or if no new error, the value
                of trx->error_state at the entry of this
                function */
    trx_t*        trx,    /* in: transaction */
    que_thr_t*    thr,    /* in: query thread */
    trx_savept_t*    savept)    /* in: savepoint or NULL */
{
...
else if (err == DB_DEADLOCK //发生死锁
           || err == DB_LOCK_TABLE_FULL
           || (err == DB_LOCK_WAIT_TIMEOUT
               && row_rollback_on_timeout)) {
        /* Roll back the whole transaction; this resolution was added
        to version 3.23.43 */

        trx_general_rollback_for_mysql(trx, FALSE, NULL); //事务全部回滚

    } else if (err == DB_OUT_OF_FILE_SPACE
           || err == DB_LOCK_WAIT_TIMEOUT) {

        ut_ad(!(err == DB_LOCK_WAIT_TIMEOUT
                && row_rollback_on_timeout));

               if (savept) { //回滚到上一个保存点
            /* Roll back the latest, possibly incomplete
            insertion or update */

            trx_general_rollback_for_mysql(trx, TRUE, savept);
        }
        /* MySQL will roll back the latest SQL statement */
...