| 知乎专栏 ｜ 多维度架构 |
目录
COMMIT - save work done SAVEPOINT - identify a point in a transaction to which you can later roll back ROLLBACK - restore database to original since the last COMMIT SET TRANSACTION - Change transaction options like what rollback segment to use
锁机制
1) 共享锁：由读表操作加上的锁，加锁后其他用户只能获取该表或行的共享锁，不能获取排它锁，也就是说只能读不能写
2) 排它锁：由写表操作加上的锁，加锁后其他用户不能获取该表或行的任何锁，典型是mysql事务中的
锁的范围:
行锁: 对某行记录加上锁
表锁: 对整个表加上锁
共享锁(share mode), 排他锁(for update)
下面做作一个实验，验证锁的效果
终端一,首先进入事务状态然后运行下面语句
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from t1 where id='3' for update; +----+--------+---------------------+---------------------+ | id | name | ctime | mtime | +----+--------+---------------------+---------------------+ | 3 | test | 0000-00-00 00:00:00 | 2013-01-14 13:05:41 | +----+--------+---------------------+---------------------+ 1 row in set (0.00 sec)
终端二, 查询表中数据
mysql> select * from t1; +----+--------+---------------------+---------------------+ | id | name | ctime | mtime | +----+--------+---------------------+---------------------+ | 1 | neo | 0000-00-00 00:00:00 | 2013-01-14 13:00:00 | | 2 | zen | 0000-00-00 00:00:00 | 2013-01-14 13:00:43 | | 3 | test | 0000-00-00 00:00:00 | 2013-01-14 13:05:41 | +----+--------+---------------------+---------------------+ 3 rows in set (0.00 sec)
增加“for update”查询非锁定记录
mysql> select * from t1 where id=2 for update; +----+------+---------------------+---------------------+ | id | name | ctime | mtime | +----+------+---------------------+---------------------+ | 2 | zen | 0000-00-00 00:00:00 | 2013-01-14 13:00:43 | +----+------+---------------------+---------------------+ 1 row in set (0.00 sec)
查询被锁定记录
mysql> select * from t1 where id=3 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
查询所有记录，因为记录中包含了id=3那条，所以也不允许查询。
mysql> select * from t1 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
测试修改记录
mysql> UPDATE `t1` SET `name`='testaa' WHERE `id`=3; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
![[提示]](/graphics/tip.png) | 提示 |
|---|---|
在没有出现ERROR 1205 (HY000)的这段时间，只要终端一中执行commit,rollback锁就释放了.终端二中的语句就会运行。 select trx_query from information_schema.innodb_trx; 可以查看被锁的SQL语句 |
LOCK TABLES tablename WRITE; LOCK TABLES tablename READ; ... ... UNLOCK TABLES;
CREATE TABLE `locking` (
`name` VARCHAR(50) NULL DEFAULT NULL
)
ENGINE=InnoDB
;
mysql> insert into locking values('test');
Query OK, 1 row affected (0.02 sec)
mysql> select * from locking;
+------+
| name |
+------+
| test |
+------+
1 row in set (0.00 sec)
mysql> UNLOCK TABLES;
mysql> LOCK TABLES locking READ;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into locking values('test');
ERROR 1099 (HY000): Table 'locking' was locked with a READ lock and can't be updated
mysql> LOCK TABLE locking WRITE;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from locking;
+------+
| name |
+------+
| test |
| test |
+------+
2 rows in set (0.00 sec)
mysql> insert into locking values('test');
Query OK, 1 row affected (0.05 sec)
mysql> UNLOCK TABLES;
mysql> LOCK TABLE locking AS myalias READ; Query OK, 0 rows affected (0.00 sec) mysql> select * from locking; ERROR 1100 (HY000): Table 'locking' was not locked with LOCK TABLES mysql> select * from locking as myalias; +------+ | name | +------+ | test | | test | | test | +------+ 3 rows in set (0.00 sec)
当你开启了事务 begin 忘记，或者各种原因没有commit也没有rollback。悲剧了！
begin; SET SESSION wait_timeout = 60; select * from locking for update;
60秒内如果没有commit/rollback将自动释放本次事务。
使用 for update 是会遇到一个问题，就是其他用户会漫长的等待，而我们需要程序非阻塞运行，当遇到 for update 的时候应该立即返回此表已被加锁。
mysql 并没有实现 nowait 关键字（类似Oracle的功能），但又一个方法能够达到同样目的。
mysql> select @@innodb_version; +------------------+ | @@innodb_version | +------------------+ | 5.6.24 | +------------------+ 1 row in set (0.05 sec) mysql> select * from locking; ERROR 1100 (HY000): Table 'locking' was not locked with LOCK TABLES
此时需要等待很长时间才能提示 “Table 'locking' was not locked with LOCK TABLES”
mysql> set session innodb_lock_wait_timeout=1; Query OK, 0 rows affected (0.02 sec) mysql> select * from locking for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
设置 innodb_lock_wait_timeout 参数后，很快就返回
mysql> show variables like 'innodb_lock_wait_timeout'; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | innodb_lock_wait_timeout | 1 | +--------------------------+-------+ 1 row in set (0.00 sec) mysql> show global variables like 'innodb_lock_wait_timeout'; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | innodb_lock_wait_timeout | 50 | +--------------------------+-------+ 1 row in set (0.00 sec)
innodb_lock_wait_timeout 默认值是 50