LOCK TABLES

http://blog.csdn.net/zyz511919766/article/details/16342003

http://blog.csdn.net/zyz511919766/article/category/1095365

1语法

LOCK TABLES tbl_name[[AS] alias] lock_type [, tbl_name [[AS] alias] lock_type] ...

lock_type:READ[LOCAL]| [LOW_PRIORITY] WRITE

UNLOCK TABLES

2简单使用

一个session只能为自己获取锁和释放锁，不能为其他session获取锁，也不能释放由其他session保持的锁。

要为一个对象获取锁，需具备该对象上的SELECT权限和LOCK TABLES权限。LOCK TABLES语句为当前session显式的获取表锁。可以为tables获取锁，也可以为view，trigger获取锁。对于为view加锁，LOCK TABLES语句会为view中使用的所有基表加锁，对于trigger同样如此。UNLOCK TABLES显示的释放当前session锁保持的锁。另外通过LOCK TABLEs语句为当前session获取新锁前会隐式的释放当前session之前的所有锁。

mysql> show open tables from test;
+----------+---------------------+--------+-------------+
| Database | Table               | In_use | Name_locked |
+----------+---------------------+--------+-------------+
| test     | rr                  |      0 |           0 |
| test     | innodb_lock_monitor |      0 |           0 |
| test     | t                   |      0 |           0 |
| test     | t1                  |      0 |           0 |
| test     | user                |      0 |           0 |
+----------+---------------------+--------+-------------+
5 rows in set (0.00 sec)

mysql> lock table t read;
Query OK, 0 rows affected (0.00 sec)

mysql> show open tables from test;
+----------+---------------------+--------+-------------+
| Database | Table               | In_use | Name_locked |
+----------+---------------------+--------+-------------+
| test     | rr                  |      0 |           0 |
| test     | innodb_lock_monitor |      0 |           0 |
| test     | t                   |      1 |           0 |
| test     | t1                  |      0 |           0 |
| test     | user                |      0 |           0 |
+----------+---------------------+--------+-------------+
5 rows in set (0.01 sec)

mysql> lock table t1 read;
Query OK, 0 rows affected (0.00 sec)

mysql> show open tables from test;  表t的锁释放
+----------+---------------------+--------+-------------+
| Database | Table               | In_use | Name_locked |
+----------+---------------------+--------+-------------+
| test     | rr                  |      0 |           0 |
| test     | innodb_lock_monitor |      0 |           0 |
| test     | t                   |      0 |           0 |
| test     | t1                  |      1 |           0 |
| test     | user                |      0 |           0 |
+----------+---------------------+--------+-------------+
5 rows in set (0.01 sec)

mysql> insert into t select 2;
ERROR 1100 (HY000): Table 't' was not locked with LOCK TABLES

mysql> insert into t1 select 10,10;
ERROR 1099 (HY000): Table 't1' was locked with a READ lock and can't be updat

mysql> insert into rr select 10;
ERROR 1100 (HY000): Table 'rr' was not locked with LOCK TABLES

会话2：

mysql> insert into t1 select 10,10;
^CCtrl-C -- sending "KILL QUERY 2" to server ...
Ctrl-C -- query aborted.
ERROR 1317 (70100): Query execution was interrupted
mysql> insert into t select 10,10;
Query OK, 1 row affected (0.18 sec)
Records: 1  Duplicates: 0  Warnings: 0

UNLOCK TABLES还还以用来释放FLUSH TABLES WITH READ LOCKS获取的全局锁，该锁锁定所有库的所有表。（稍后详细介绍）

对非临时表执行LOCK TABLES不会影响对临时表的访问。因为该类表可以且仅可以由创建它的session访问且不管什么类型的锁都对其没有影响。

mysql> create temporary table tmp like t;
Query OK, 0 rows affected (0.22 sec)

mysql> show create table  tmp;

CREATE TEMPORARY TABLE `tmp` (
  `id` int(11) DEFAULT NULL,
  `val` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 

mysql> select * from tmp;
Empty set (0.00 sec)

mysql> lock table t read;
Query OK, 0 rows affected (0.01 sec)

mysql> insert into rr select 10,10;
ERROR 1100 (HY000): Table 'rr' was not locked with LOCK TABLES
mysql> insert into tmp select 10,10;      临时表可以插入
Query OK, 1 row affected (0.18 sec)
Records: 1  Duplicates: 0  Warnings: 0

但是对临时表执行LOCK TABLES，可以对其他非临时表产生影响。

mysql> unlock tables;
Query OK, 0 rows affected (0.00 sec)

mysql> lock table tmp read;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from r;
ERROR 1100 (HY000): Table 'r' was not locked with LOCK TABLES

mysql> select * from tmp;
+------+------+
| id   | val  |
+------+------+
|   10 |   10 |
+------+------+
1 row in set (0.00 sec)

mysql> insert into rr select 100,100;                               非临时表也产生影响了
ERROR 1100 (HY000): Table 'rr' was not locked with LOCK TABLES

mysql> insert into tmp select 100,100;                              临时表照样可以插入
Query OK, 1 row affected (0.17 sec)
Records: 1  Duplicates: 0  Warnings: 0

3获取锁的规则

有下列可用的锁类型

Ø  READ[LOCAL] lock

ü  保持该锁的session可以读取加锁的表，但不能写，对于没有加锁的表不能读也不能写。

会话1：

mysql> lock table rr read;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from rr;
+------+------+------+------+------+------+
| a    | xx   | xxx  | xxxx | zz   | rr   |
+------+------+------+------+------+------+
|    1 | NULL | NULL | NULL | NULL | NULL |
+------+------+------+------+------+------+
1 row in set (0.17 sec)

mysql> select * from t;   //没加锁的表也不能读与写
ERROR 1100 (HY000): Table 'r' was not locked with LOCK TABLES

mysql> insert into rr select 1,2,3,4,5,6;
ERROR 1099 (HY000): Table 'rr' was locked with a READ lock and can't be updated

另外一个会话是可以查询的，插入的 ，但rr 表不可插入

mysql> select * from t;
+------+------+
| id   | val  |
+------+------+
|    1 |    3 |
|    2 |    4 |
|    5 |    6 |
|    5 |    6 |
|    8 |    9 |
|    5 |    6 |
|    8 |    9 |
+------+------+
7 rows in set (0.00 sec)

mysql> insert into t select 100,100;
Query OK, 1 row affected (0.18 sec)
Records: 1  Duplicates: 0  Warnings: 0

ü  多个session可同时为同一个表获取READ锁

session1

mysql> lock tables rr read;
Query OK, 0 rows affected (0.00 sec)

mysql> show open tables from test;
+----------+---------------------+--------+-------------+
| Database | Table               | In_use | Name_locked |
+----------+---------------------+--------+-------------+
| test     | rr                  |      1 |           0 |
| test     | innodb_lock_monitor |      0 |           0 |
| test     | t                   |      0 |           0 |
| test     | t1                  |      0 |           0 |
| test     | user                |      0 |           0 |
+----------+---------------------+--------+-------------+
5 rows in set (0.01 sec)

session2

mysql>  lock tables rr read;
Query OK, 0 rows affected (0.00 sec)

mysql> show open tables from test;
+----------+---------------------+--------+-------------+
| Database | Table               | In_use | Name_locked |
+----------+---------------------+--------+-------------+
| test     | rr                  |      2 |           0 |
| test     | innodb_lock_monitor |      0 |           0 |
| test     | t                   |      0 |           0 |
| test     | t1                  |      0 |           0 |
| test     | user                |      0 |           0 |
+----------+---------------------+--------+-------------+
5 rows in set (0.01 sec)

session3

mysql> lock tables rr read;
Query OK, 0 rows affected (0.00 sec)

mysql> show open tables from test;
+----------+---------------------+--------+-------------+
| Database | Table               | In_use | Name_locked |
+----------+---------------------+--------+-------------+
| test     | rr                  |      3 |           0 |
| test     | innodb_lock_monitor |      0 |           0 |
| test     | t                   |      0 |           0 |
| test     | t1                  |      0 |           0 |
| test     | user                |      0 |           0 |
+----------+---------------------+--------+-------------+
5 rows in set (0.01 sec)

ü  当前session只能读取明确获取了READ锁的表，不能更新该表，也不能读取和更新其他没有获取READ锁的表。其他的session可以读取没有在当前session中明确获取READ锁的表，当然也可以读取获取了READ锁的表。但是更新在其他session中获取了READ锁的表会被阻塞，可以正常更新其他的表。

session1(当前session)

mysql> select * from rr;
+------+------+------+------+------+------+
| a    | xx   | xxx  | xxxx | zz   | rr   |
+------+------+------+------+------+------+
|    1 | NULL | NULL | NULL | NULL | NULL |
+------+------+------+------+------+------+
1 row in set (0.01 sec)

mysql> select * from t;
ERROR 1100 (HY000): Table 't' was not locked with LOCK TABLES
mysql> insert into rr select 1,2,3,4,5,6;
ERROR 1099 (HY000): Table 'rr' was locked with a READ lock and can't be updated

session2

mysql> select * from rr;
+------+------+------+------+------+------+
| a    | xx   | xxx  | xxxx | zz   | rr   |
+------+------+------+------+------+------+
|    1 | NULL | NULL | NULL | NULL | NULL |
+------+------+------+------+------+------+
1 row in set (0.01 sec)

mysql>  select * from t;
+------+------+
| id   | val  |
+------+------+
|    1 |    3 |
|    2 |    4 |
|    5 |    6 |
|    5 |    6 |
|    8 |    9 |
|    5 |    6 |
|    8 |    9 |
|  100 |  100 |
+------+------+
8 rows in set (0.01 sec)

mysql> insert into rr select 1,2,3,4,5,6;
等待

mysql> insert into rr select 1,2,3,4,5,6;
^CCtrl-C -- sending "KILL QUERY 2" to server ...
Ctrl-C -- query aborted.
ERROR 1317 (70100): Query execution was interrupted
mysql> insert into t select 1,2;
Query OK, 1 row affected (0.19 sec)
Records: 1  Duplicates: 0  Warnings: 0

LOCAL修饰符可以允许在其他session中对在当前session中获取了READ锁的的表执行插入。但是当在保持锁时若使用server外的线程来操纵数据库则不能使用READ LOCAL。另外，对于InnoDB表，READ LOCAL与READ相同。

Ø  [LOW_PRIORITY]WRITE LOCK

ü  保持该锁的session可读/写获取了WRITE锁的表，但不可以读/写其他表。其他的session可以读取和更新没有在当前session中获取WRITE锁的表，但其他session在读取和更新在当前session中获取了WRITE锁的表时会阻塞。

mysql> lock table rr write;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from t;
ERROR 1100 (HY000): Table 't' was not locked with LOCK TABLES
mysql> insert into rr select 1,2,3,4,5,6;
Query OK, 1 row affected (0.19 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> insert into t select 1,2;
ERROR 1100 (HY000): Table 't' was not locked with LOCK TABLES

不能同时在多个session中为同一个table获得WRITE锁。

mysql> lock table rr write;
等待

LOW_PRIORITY修饰符用于之前版本的MySQL，影响锁定行为。但在MySQL5.6.5之后不再使用该选项。

需在单条LOCK TABLES语句中获取所有所需的锁，因为每次重新执行LOCK TABLES语句会释放之前获取的锁。一旦保持了获取的锁，该session只能访问锁定的表。但是，INFORMATION_SCHEMA数据库是个例外。

不能在单条查询中使用相同的名称多次引用锁定的表。需使用别名，且为别名获取单独的锁。可以看到会在a表上加两个锁。第一次执行INSERT因为包含了对锁定的表的同一名称的多次引用而发生错误。第二次执行INSERT操作使用了别名不会发生上述问题（这里显示的问题是主键冲突的问题）

mysql> show open tables from test;
+----------+---------------------+--------+-------------+
| Database | Table               | In_use | Name_locked |
+----------+---------------------+--------+-------------+
| test     | rr                  |      0 |           0 |
| test     | innodb_lock_monitor |      0 |           0 |
| test     | t                   |      0 |           0 |
| test     | t1                  |      0 |           0 |
| test     | user                |      0 |           0 |
+----------+---------------------+--------+-------------+
5 rows in set (0.00 sec)

mysql> lock table rr write ,rr as a1 read;
Query OK, 0 rows affected (0.00 sec)

mysql> show open tables from test;
+----------+---------------------+--------+-------------+
| Database | Table               | In_use | Name_locked |
+----------+---------------------+--------+-------------+
| test     | rr                  |      2 |           0 |
| test     | innodb_lock_monitor |      0 |           0 |
| test     | t                   |      0 |           0 |
| test     | t1                  |      0 |           0 |
| test     | user                |      0 |           0 |
+----------+---------------------+--------+-------------+
5 rows in set (0.00 sec)

mysql> insert into rr select * from rr;
ERROR 1100 (HY000): Table 'rr' was not locked with LOCK TABLES

mysql> insert into rr select * from rr as a1;
Query OK, 2 rows affected (0.02 sec)
Records: 2  Duplicates: 0  Warnings: 0

如果语句通过别名引用表，那么锁定表时需使用相同的别名。

mysql> lock table rr read;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from a as a1;
ERROR 1100 (HY000): Table 'a1' was not locked with LOCK TABLES
mysql> lock table rr as a1 read;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from  rr as a1;
+------+------+------+------+------+------+
| a    | xx   | xxx  | xxxx | zz   | rr   |
+------+------+------+------+------+------+
|    1 | NULL | NULL | NULL | NULL | NULL |
|    1 |    2 |    3 |    4 |    5 |    6 |
|    1 | NULL | NULL | NULL | NULL | NULL |
|    1 |    2 |    3 |    4 |    5 |    6 |
+------+------+------+------+------+------+
4 rows in set (0.01 sec)

WRITE lock优先级通常高于READ lock，这意味着若一个session为表获取了一个READ锁，之后另一session为该表请求WRITE锁，那么后续的READ锁请求会一直等到请求WRITE锁的session获取锁并释放锁后才能满足。

LOCKTABLE语句获取锁的过程

Ø  按内部定义的顺序排序将要被锁定的表

Ø  若一个表将要获取READ和WRITE锁，则将WRITE锁请求放在READ锁请求前

Ø  逐个锁定表

释放锁的规则

Ø  一个session锁保持的锁被释放时所有的锁同时全部释放。

Ø  session可通过UNLOCK TABLES语句明确的释放其保持的锁。

Ø  若已经保持有锁的session通过LOCK TABLES语句来获得新的锁，那么原来已经存在的锁在新锁获得前会被隐式释放。

Ø  若session开启了事务，那么会隐式的执行UNLOCK TABLES语句释放其所有的锁。

Ø  若session连接断开，无论是正常断开还是异常断开，服务器会隐式的释放该session保持的所有锁。即使客户端重新连接也不会再重新获得这些锁。

Ø  若在锁定的表上使用ALTER TABLE语句，则该表会变为未锁定的。

参考

http://dev.mysql.com/doc/refman/5.6/en/lock-tables.html