mysql
由于InnoDB预设是Row-Level Lock,所以只有「明确」的指定主键,MySQL才会执行Row lock (只锁住被选取的资料例) ,否则MySQL将会执行Table Lock (将整个资料表单给锁住)。
t
id int(11) NOT NULL DEFAULT '0',
name varchar(255) DEFAULT NULL,
id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
例1: (明确指定主键,并且有此笔资料,row lock) 窗口一:
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from t where id = 1 for update;
id | name |
1 | bingo |
1 row in set (0.00 sec)
窗口二:
mysql> select * from t where id = 1;
id | name |
1 | bingo |
1 row in set (0.01 sec)
mysql> update t set name = 'xxm' where id = 2;
Query OK, 1 row affected
(0.08 sec) Rows matched: 1 Changed: 1 Warnings: 0
mysql> update t set name = 'icey' where id = 1;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
由此可见,当明确指定主键,并且有此资料时,锁的是where后面的记录,即这里的id= 1; 接下来来看下没有此资料的情况下会不会被锁。
例2: (明确指定主键,若查无此笔资料,无lock)
窗口1:
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from t where id = 11 for update;
Empty set (0.00 sec)
窗口2:
mysql> update t set name = 'qweq' where id = 1;
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update t set name = 'qw' where id = 2;
Query OK, 1 row affected (0.06 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update t set name = 'vqw' where id = 3;
Query OK, 1 row affected (0.05 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update t set name = 'vqws' where id = 4;
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update t set name = 'vqs' where id = 5;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update t set name = 'vqs' where id = 11;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0
mysql> select * from t;
+----+------+
id | name |
1 | qweq |
2 | qw |
3 | vqw |
4 | vqws |
5 | vqs |
由此得出结论,在没有此资料的情况下,即使你for update也是不锁的
例2: (无主键,table lock)
窗口1:
mysql> select * from t where name ='qweq' for update;
id | name |
1 | qweq |
1 row in set (0.00 sec)
窗口2:
mysql> update t set name = 'vqs' where id = 1;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t set name = 'vqs' where id = 2;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
由此可见,没有明确主键的情况下锁全表;
例3: (主键不明确,table lock)
SELECT * FROM products WHERE id<>'3' FOR UPDATE;
窗口1: mysql> select * from t where id <> 2 for update;
id | name |
1 | qweq |
3 | vqw |
4 | vqws |
5 | vqs |
4 rows in set (0.00 sec)
窗口2:
mysql> update t set name = 'vqs' where id = 1; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t set name = 'vqs' where id = 2;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
在主键不明确的情况下,锁全表
例4: (主键不明确,table lock)
窗口1:
mysql> begin; Query OK, 0 rows affected (0.00 sec)
mysql> select * from t where id like 3 for update;
id | name |
3 | vqw |
1 row in set (0.00 sec)
窗口2:
mysql> update t set name = 'vqs' where id = 1;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t set name = 'vqs' where id = 2;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
锁全表
注1: FOR UPDATE仅适用于InnoDB,且必须在交易区块(BEGIN/COMMIT)中才能生效。
oracle
SELECT...FOR UPDATE 语句的语法如下:
SELECT ... FOR UPDATE [OF column_list][WAIT n|NOWAIT][SKIP LOCKED];
其中:
OF 子句用于指定即将更新的列,即锁定行上的特定列。
WAIT 子句指定等待其他用户释放锁的秒数,防止无限期的等待。
“使用FOR UPDATE WAIT”子句的优点如下:
1防止无限期地等待被锁定的行;
2允许应用程序中对锁的等待时间进行更多的控制。
3对于交互式应用程序非常有用,因为这些用户不能等待不确定
4 若使用了skip locked,则可以越过锁定的行,不会报告由wait n 引发的‘资源忙’异常报告
示例:
create table t(a varchar2(20),b varchar2(20));
insert into t values('1','1');
insert into t values('2','2');
insert into t values('3','3');
insert into t values('4','4');
现在执行如下操作:
在plsql develope中打开两个sql窗口,
在1窗口中运行sql
select * from t where a='1' for update;
在2窗口中运行sql1
1. select * from t where a='1'; 这一点问题也没有,因为行级锁不会影响纯粹的select语句
再运行sql2
2. select * from t where a='1' for update; 则这一句sql在执行时,永远处于等待状态,除非窗口1中sql被提交或回滚。
如何才能让sql2不等待或等待指定的时间呢? 我们再运行sql3
3. select * from t where a='1' for update nowait; 则在执行此sql时,直接报资源忙的异常。
若执行 select * from t where a='1' for update wait 6; 则在等待6秒后,报 资源忙的异常。
如果我们执行sql4
4. select * from t where a='1' for update nowait skip Locked; 则执行sql时,即不等待,也不报资源忙异常。
现在我们看看执行如下操作将会发生什么呢?
在窗口1中执行:
select * from t where rownum<=3 nowait skip Locked;
在窗口2中执行:
select * from t where rownum<=6 nowait skip Locked;
select for update 也就如此了吧,insert、update、delete操作默认加行级锁,其原理和操作与select for update并无两样。
select for update of,这个of子句在牵连到多个表时,具有较大作用,如不使用of指定锁定的表的列,则所有表的相关行均被锁定,若在of中指定了需修改的列,则只有与这些列相关的表的行才会被锁定。