select for update 对表施加的锁模式分两种情况:
只有当where 条件中明确地使用指定主键时,MySQL 才会对表执行Row lock (只锁住被选取的数据) ,
否则MySQL 将会执行Table Lock (将整个数据表单给锁住)。
下面以具体的实例验证上面的结论：
注意 表mail_queue 的结构如下
root@127.0.0.1 : test 22:06:52> show create table mail_queue \G
*************************** 1. row ***************************
Table: mail_queue
Create Table: CREATE TABLE `mail_queue` (
`mail_id` int(11) NOT NULL AUTO_INCREMENT,
`is_sent` tinyint(4) NOT NULL COMMENT '是否已经发送(-1,失败，0没发送，1发送成功)',
`mail_title` varchar(127) NOT NULL COMMENT '邮件标题',
`mail_from` varchar(127) NOT NULL COMMENT '邮件发件人',
`mail_to` varchar(127) NOT NULL COMMENT '邮件接收人',
`mail_content` text NOT NULL COMMENT '邮件内容',
`add_time` int(11) NOT NULL COMMENT '?'
`send_time` int(11) NOT NULL COMMENT '?'
`agent_id` int(11) NOT NULL COMMENT '?',
`service_id` int(11) NOT NULL COMMENT 'ID',
PRIMARY KEY (`mail_id`),
KEY `ind_mq_atime_isent_mail_id` (`add_time`,`is_sent`,`mail_id`),
KEY `sid` (`service_id`)
) ENGINE=InnoDB AUTO_INCREMENT=7391601 DEFAULT CHARSET=utf8 COMMENT='邮件队列'
1 row in set (0.00 sec)
例1: (明确指定主键,并且有此数据,row lock)
session1
root@127.0.0.1 : test 21:50:40> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)
root@127.0.0.1 : test 21:50:56>
root@127.0.0.1 : test 21:50:56> select mail_id from mail_queue where mail_id=237 for update;
+---------+
| mail_id |
+---------+
| 237 |
+---------+
1 row in set (0.00 sec)
session2
root@127.0.0.1 : test 21:51:42> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)
root@127.0.0.1 : test 21:51:48> select mail_id from mail_queue where mail_id=237;
+---------+
| mail_id |
+---------+
| 237 |
+---------+
1 row in set (0.00 sec)
例2: (明确指定主键,若查无此数据,无lock)
session 1
root@127.0.0.1 : test 22:06:05> select mail_id from mail_queue where mail_id=-1 for update;
Empty set (0.00 sec)
session 2
root@127.0.0.1 : test 22:06:45> select mail_id,is_sent from mail_queue where mail_id=237 for update;
+---------+---------+
| mail_id | is_sent |
+---------+---------+
| 237 | 1 |
+---------+---------+
1 row in set (0.00 sec)
例3: (无主键,table lock)
mail_to 非主键
session 1
root@127.0.0.1 : test 22:09:20> select mail_to from mail_queue where mail_to='slxx_721521@126.com' for update;
+---------------------+
| mail_to |
+---------------------+
| slxx_721521@126.com |
| slxx_721521@126.com |
| slxx_721521@126.com |
| slxx_721521@126.com |
| slxx_721521@126.com |
| slxx_721521@126.com |
| slxx_721521@126.com |
| slxx_721521@126.com |
| slxx_721521@126.com |
| slxx_721521@126.com |
+---------------------+
10 rows in set (50.23 sec)
session 2 查询被阻塞
root@127.0.0.1 : test 22:10:02> select mail_id,is_sent from mail_queue where mail_id=237 for update;
Ctrl-C -- sending "KILL QUERY 38535" to server ...
Ctrl-C -- query aborted.
ERROR 1317 (70100): Query execution was interrupted
root@127.0.0.1 : test 22:11:30>
例4: (主键不明确,table lock)
session 1
root@127.0.0.1 : test 22:11:48> select count(1) from mail_queue where mail_id <> 100000 for update;
+----------+
| count(1) |
+----------+
| 3695458 |
+----------+
1 row in set (27.45 sec)
session 2 会话被锁
root@127.0.0.1 : test 22:12:35> select mail_id,is_sent from mail_queue where mail_id=237 for update;
Ctrl-C -- sending "KILL QUERY 38535" to server ...
Ctrl-C -- query aborted.
ERROR 1317 (70100): Query execution was interrupted
root@127.0.0.1 : test 22:13:10>
例5: (主键不明确,table lock)
session 1
root@127.0.0.1 : test 22:13:46> select count(1) from mail_queue where mail_id like '100000' for update;
+----------+
| count(1) |
+----------+
| 0 |
+----------+
1 row in set (24.31 sec)
root@127.0.0.1 : test 22:14:22>
session 2
root@127.0.0.1 : test 22:13:34> select mail_id,is_sent from mail_queue where mail_id=237 for update;
Ctrl-C -- sending "KILL QUERY 38535" to server ...
Ctrl-C -- query aborted.
ERROR 1317 (70100): Query execution was interrupted
注:FOR UPDATE 仅适用于InnoDB,且必须在事务区块(BEGIN/COMMIT)中才能生效。