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请求锁模式 是否兼容 当前锁模式 | None | 读锁 | 写锁 |
读锁 | 是 | 是 | 否 |
写锁 | 是 | 否 | 否 |
session_1 | session_2 |
获得表film_text的WRITE锁定 mysql> lock table film_text write; Query OK, 0 rows affected (0.00 sec) | |
当前session对锁定表的查询、更新、插入操作都可以执行: mysql> select film_id,title from film_text where film_id = 1001; +---------+-------------+ | film_id | title | +---------+-------------+ | 1001 | Update Test | +---------+-------------+ 1 row in set (0.00 sec) mysql> insert into film_text (film_id,title) values(1003,'Test'); Query OK, 1 row affected (0.00 sec) mysql> update film_text set title = 'Test' where film_id = 1001; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 | 其他session对锁定表的查询被阻塞,需要等待锁被释放: mysql> select film_id,title from film_text where film_id = 1001; 等待 |
释放锁: mysql> unlock tables; Query OK, 0 rows affected (0.00 sec) | 等待 |
Session2获得锁,查询返回: mysql> select film_id,title from film_text where film_id = 1001; +---------+-------+ | film_id | title | +---------+-------+ | 1001 | Test | +---------+-------+ 1 row in set (57.59 sec) |
session_1 | session_2 |
获得表film_text的READ锁定 mysql> lock table film_text read; Query OK, 0 rows affected (0.00 sec) | |
当前session可以查询该表记录 mysql> select film_id,title from film_text where film_id = 1001; +---------+------------------+ | film_id | title | +---------+------------------+ | 1001 | ACADEMY DINOSAUR | +---------+------------------+ 1 row in set (0.00 sec) | 其他session也可以查询该表的记录 mysql> select film_id,title from film_text where film_id = 1001; +---------+------------------+ | film_id | title | +---------+------------------+ | 1001 | ACADEMY DINOSAUR | +---------+------------------+ 1 row in set (0.00 sec) |
当前session不能查询没有锁定的表 mysql> select film_id,title from film where film_id = 1001; ERROR 1100 (HY000): Table 'film' was not locked with LOCK TABLES | 其他session可以查询或者更新未锁定的表 mysql> select film_id,title from film where film_id = 1001; +---------+---------------+ | film_id | title | +---------+---------------+ | 1001 | update record | +---------+---------------+ 1 row in set (0.00 sec) mysql> update film set title = 'Test' where film_id = 1001; Query OK, 1 row affected (0.04 sec) Rows matched: 1 Changed: 1 Warnings: 0 |
当前session中插入或者更新锁定的表都会提示错误: mysql> insert into film_text (film_id,title) values(1002,'Test'); ERROR 1099 (HY000): Table 'film_text' was locked with a READ lock and can't be updated mysql> update film_text set title = 'Test' where film_id = 1001; ERROR 1099 (HY000): Table 'film_text' was locked with a READ lock and can't be updated | 其他session更新锁定表会等待获得锁: mysql> update film_text set title = 'Test' where film_id = 1001; 等待 |
释放锁 mysql> unlock tables; Query OK, 0 rows affected (0.00 sec) | 等待 |
Session获得锁,更新操作完成: mysql> update film_text set title = 'Test' where film_id = 1001; Query OK, 1 row affected (1 min 0.71 sec) Rows matched: 1 Changed: 1 Warnings: 0 |
session_1 | session_2 |
获得表film_text的READ LOCAL锁定 mysql> lock table film_text read local; Query OK, 0 rows affected (0.00 sec) | |
当前session不能对锁定表进行更新或者插入操作: mysql> insert into film_text (film_id,title) values(1002,'Test'); ERROR 1099 (HY000): Table 'film_text' was locked with a READ lock and can't be updated mysql> update film_text set title = 'Test' where film_id = 1001; ERROR 1099 (HY000): Table 'film_text' was locked with a READ lock and can't be updated | 其他session可以进行插入操作,但是更新会等待: mysql> insert into film_text (film_id,title) values(1002,'Test'); Query OK, 1 row affected (0.00 sec) mysql> update film_text set title = 'Update Test' where film_id = 1001; 等待 |
当前session不能访问其他session插入的记录: mysql> select film_id,title from film_text where film_id = 1002; Empty set (0.00 sec) | |
释放锁: mysql> unlock tables; Query OK, 0 rows affected (0.00 sec) | 等待 |
当前session解锁后可以获得其他session插入的记录: mysql> select film_id,title from film_text where film_id = 1002; +---------+-------+ | film_id | title | +---------+-------+ | 1002 | Test | +---------+-------+ 1 row in set (0.00 sec) | Session2获得锁,更新操作完成: mysql> update film_text set title = 'Update Test' where film_id = 1001; Query OK, 1 row affected (1 min 17.75 sec) Rows matched: 1 Changed: 1 Warnings: 0 |
读数据一致性及允许的并发副作用 隔离级别 | 读数据一致性 | 脏读 | 不可重复读 | 幻读 |
未提交读(Read uncommitted) | 最低级别,只能保证不读取物理上损坏的数据 | 是 | 是 | 是 |
已提交度(Read committed) | 语句级 | 否 | 是 | 是 |
可重复读(Repeatable read) | 事务级 | 否 | 否 | 是 |
可序列化(Serializable) | 最高级别,事务级 | 否 | 否 | 否 |
请求锁模式 是否兼容 当前锁模式 | X | IX | S | IS |
X | 冲突 | 冲突 | 冲突 | 冲突 |
IX | 冲突 | 兼容 | 冲突 | 兼容 |
S | 冲突 | 冲突 | 兼容 | 兼容 |
IS | 冲突 | 兼容 | 兼容 | 兼容 |
session_1 | session_2 |
mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE | +----------+------------+-----------+ 1 row in set (0.00 sec) | mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE | +----------+------------+-----------+ 1 row in set (0.00 sec) |
当前session对actor_id=178的记录加share mode 的共享锁: mysql> select actor_id,first_name,last_name from actor where actor_id = 178 lock in share mode; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE | +----------+------------+-----------+ 1 row in set (0.01 sec) | |
其他session仍然可以查询记录,并也可以对该记录加share mode的共享锁: mysql> select actor_id,first_name,last_name from actor where actor_id = 178 lock in share mode; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE | +----------+------------+-----------+ 1 row in set (0.01 sec) | |
当前session对锁定的记录进行更新操作,等待锁: mysql> update actor set last_name = 'MONROE T' where actor_id = 178; 等待 | |
其他session也对该记录进行更新操作,则会导致死锁退出: mysql> update actor set last_name = 'MONROE T' where actor_id = 178; ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction | |
获得锁后,可以成功更新: mysql> update actor set last_name = 'MONROE T' where actor_id = 178; Query OK, 1 row affected (17.67 sec) Rows matched: 1 Changed: 1 Warnings: 0 |
session_1 | session_2 |
mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE | +----------+------------+-----------+ 1 row in set (0.00 sec) | mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE | +----------+------------+-----------+ 1 row in set (0.00 sec) |
当前session对actor_id=178的记录加for update的排它锁: mysql> select actor_id,first_name,last_name from actor where actor_id = 178 for update; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE | +----------+------------+-----------+ 1 row in set (0.00 sec) | |
其他session可以查询该记录,但是不能对该记录加共享锁,会等待获得锁: mysql> select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE | +----------+------------+-----------+ 1 row in set (0.00 sec) mysql> select actor_id,first_name,last_name from actor where actor_id = 178 for update; 等待 | |
当前session可以对锁定的记录进行更新操作,更新后释放锁: mysql> update actor set last_name = 'MONROE T' where actor_id = 178; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> commit; Query OK, 0 rows affected (0.01 sec) | |
其他session获得锁,得到其他session提交的记录: mysql> select actor_id,first_name,last_name from actor where actor_id = 178 for update; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178 | LISA | MONROE T | +----------+------------+-----------+ 1 row in set (9.59 sec) |
session_1 | session_2 |
mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) mysql> select * from tab_no_index where id = 1 ; +------+------+ | id | name | +------+------+ | 1 | 1 | +------+------+ 1 row in set (0.00 sec) | mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) mysql> select * from tab_no_index where id = 2 ; +------+------+ | id | name | +------+------+ | 2 | 2 | +------+------+ 1 row in set (0.00 sec) |
mysql> select * from tab_no_index where id = 1 for update; +------+------+ | id | name | +------+------+ | 1 | 1 | +------+------+ 1 row in set (0.00 sec) | |
mysql> select * from tab_no_index where id = 2 for update; 等待 |
session_1 | session_2 |
mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) mysql> select * from tab_with_index where id = 1 ; +------+------+ | id | name | +------+------+ | 1 | 1 | +------+------+ 1 row in set (0.00 sec) | mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) mysql> select * from tab_with_index where id = 2 ; +------+------+ | id | name | +------+------+ | 2 | 2 | +------+------+ 1 row in set (0.00 sec) |
mysql> select * from tab_with_index where id = 1 for update; +------+------+ | id | name | +------+------+ | 1 | 1 | +------+------+ 1 row in set (0.00 sec) | |
mysql> select * from tab_with_index where id = 2 for update; +------+------+ | id | name | +------+------+ | 2 | 2 | +------+------+ 1 row in set (0.00 sec) |
session_1 | session_2 |
mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) | mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) |
mysql> select * from tab_with_index where id = 1 and name = '1' for update; +------+------+ | id | name | +------+------+ | 1 | 1 | +------+------+ 1 row in set (0.00 sec) | |
虽然session_2访问的是和session_1不同的记录,但是因为使用了相同的索引,所以需要等待锁: mysql> select * from tab_with_index where id = 1 and name = '4' for update; 等待 |
session_1 | session_2 |
mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) | mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) |
mysql> select * from tab_with_index where id = 1 for update; +------+------+ | id | name | +------+------+ | 1 | 1 | | 1 | 4 | +------+------+ 2 rows in set (0.00 sec) | |
Session_2使用name的索引访问记录,因为记录没有被索引,所以可以获得锁: mysql> select * from tab_with_index where name = '2' for update; +------+------+ | id | name | +------+------+ | 2 | 2 | +------+------+ 1 row in set (0.00 sec) | |
由于访问的记录已经被session_1锁定,所以等待获得锁。: mysql> select * from tab_with_index where name = '4' for update; |
session_1 | session_2 |
mysql> select @@tx_isolation; +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) | mysql> select @@tx_isolation; +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) |
当前session对不存在的记录加for update的锁: mysql> select * from emp where empid = 102 for update; Empty set (0.00 sec) | |
这时,如果其他session插入empid为102的记录(注意:这条记录并不存在),也会出现锁等待: mysql>insert into emp(empid,...) values(102,...); 阻塞等待 | |
Session_1 执行rollback: mysql> rollback; Query OK, 0 rows affected (13.04 sec) | |
由于其他session_1回退后释放了Next-Key锁,当前session可以获得锁并成功插入记录: mysql>insert into emp(empid,...) values(102,...); Query OK, 1 row affected (13.35 sec) |
session_1 | session_2 |
mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select * from target_tab; Empty set (0.00 sec) mysql> select * from source_tab where name = '1'; +----+------+----+ | d1 | name | d2 | +----+------+----+ | 4 | 1 | 1 | | 5 | 1 | 1 | | 6 | 1 | 1 | | 7 | 1 | 1 | | 8 | 1 | 1 | +----+------+----+ 5 rows in set (0.00 sec) | mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select * from target_tab; Empty set (0.00 sec) mysql> select * from source_tab where name = '1'; +----+------+----+ | d1 | name | d2 | +----+------+----+ | 4 | 1 | 1 | | 5 | 1 | 1 | | 6 | 1 | 1 | | 7 | 1 | 1 | | 8 | 1 | 1 | +----+------+----+ 5 rows in set (0.00 sec) |
mysql> insert into target_tab select d1,name from source_tab where name = '1'; Query OK, 5 rows affected (0.00 sec) Records: 5 Duplicates: 0 Warnings: 0 | |
mysql> update source_tab set name = '1' where name = '8'; 等待 | |
commit; | |
返回结果 commit; |
session_1 | session_2 |
mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql>set innodb_locks_unsafe_for_binlog='on' Query OK, 0 rows affected (0.00 sec) mysql> select * from target_tab; Empty set (0.00 sec) mysql> select * from source_tab where name = '1'; +----+------+----+ | d1 | name | d2 | +----+------+----+ | 4 | 1 | 1 | | 5 | 1 | 1 | | 6 | 1 | 1 | | 7 | 1 | 1 | | 8 | 1 | 1 | +----+------+----+ 5 rows in set (0.00 sec) | mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select * from target_tab; Empty set (0.00 sec) mysql> select * from source_tab where name = '1'; +----+------+----+ | d1 | name | d2 | +----+------+----+ | 4 | 1 | 1 | | 5 | 1 | 1 | | 6 | 1 | 1 | | 7 | 1 | 1 | | 8 | 1 | 1 | +----+------+----+ 5 rows in set (0.00 sec) |
mysql> insert into target_tab select d1,name from source_tab where name = '1'; Query OK, 5 rows affected (0.00 sec) Records: 5 Duplicates: 0 Warnings: 0 | |
session_1未提交,可以对session_1的select的记录进行更新操作。 mysql> update source_tab set name = '8' where name = '1'; Query OK, 5 rows affected (0.00 sec) Rows matched: 5 Changed: 5 Warnings: 0 mysql> select * from source_tab where name = '8'; +----+------+----+ | d1 | name | d2 | +----+------+----+ | 4 | 8 | 1 | | 5 | 8 | 1 | | 6 | 8 | 1 | | 7 | 8 | 1 | | 8 | 8 | 1 | +----+------+----+ 5 rows in set (0.00 sec) | |
更新操作先提交 mysql> commit; Query OK, 0 rows affected (0.05 sec) | |
插入操作后提交 mysql> commit; Query OK, 0 rows affected (0.07 sec) | |
此时查看数据,target_tab中可以插入source_tab更新前的结果,这符合应用逻辑: mysql> select * from source_tab where name = '8'; +----+------+----+ | d1 | name | d2 | +----+------+----+ | 4 | 8 | 1 | | 5 | 8 | 1 | | 6 | 8 | 1 | | 7 | 8 | 1 | | 8 | 8 | 1 | +----+------+----+ 5 rows in set (0.00 sec) mysql> select * from target_tab; +------+------+ | id | name | +------+------+ | 4 | 1.00 | | 5 | 1.00 | | 6 | 1.00 | | 7 | 1.00 | | 8 | 1.00 | +------+------+ 5 rows in set (0.00 sec) | mysql> select * from tt1 where name = '1'; Empty set (0.00 sec) mysql> select * from source_tab where name = '8'; +----+------+----+ | d1 | name | d2 | +----+------+----+ | 4 | 8 | 1 | | 5 | 8 | 1 | | 6 | 8 | 1 | | 7 | 8 | 1 | | 8 | 8 | 1 | +----+------+----+ 5 rows in set (0.00 sec) mysql> select * from target_tab; +------+------+ | id | name | +------+------+ | 4 | 1.00 | | 5 | 1.00 | | 6 | 1.00 | | 7 | 1.00 | | 8 | 1.00 | +------+------+ 5 rows in set (0.00 sec) |
隔离级别 一致性读和锁 SQL | Read Uncommited | Read Commited | Repeatable Read | Serializable | |
SQL | 条件 | ||||
select | 相等 | None locks | Consisten read/None lock | Consisten read/None lock | Share locks |
范围 | None locks | Consisten read/None lock | Consisten read/None lock | Share Next-Key | |
update | 相等 | exclusive locks | exclusive locks | exclusive locks | Exclusive locks |
范围 | exclusive next-key | exclusive next-key | exclusive next-key | exclusive next-key | |
Insert | N/A | exclusive locks | exclusive locks | exclusive locks | exclusive locks |
replace | 无键冲突 | exclusive locks | exclusive locks | exclusive locks | exclusive locks |
键冲突 | exclusive next-key | exclusive next-key | exclusive next-key | exclusive next-key | |
delete | 相等 | exclusive locks | exclusive locks | exclusive locks | exclusive locks |
范围 | exclusive next-key | exclusive next-key | exclusive next-key | exclusive next-key | |
Select ... from ... Lock in share mode | 相等 | Share locks | Share locks | Share locks | Share locks |
范围 | Share locks | Share locks | Share Next-Key | Share Next-Key | |
Select * from ... For update | 相等 | exclusive locks | exclusive locks | exclusive locks | exclusive locks |
范围 | exclusive locks | Share locks | exclusive next-key | exclusive next-key | |
Insert into ... Select ... (指源表锁) | innodb_locks_unsafe_for_binlog=off | Share Next-Key | Share Next-Key | Share Next-Key | Share Next-Key |
innodb_locks_unsafe_for_binlog=on | None locks | Consisten read/None lock | Consisten read/None lock | Share Next-Key | |
create table ... Select ... (指源表锁) | innodb_locks_unsafe_for_binlog=off | Share Next-Key | Share Next-Key | Share Next-Key | Share Next-Key |
innodb_locks_unsafe_for_binlog=on | None locks | Consisten read/None lock | Consisten read/None lock | Share Next-Key |
session_1 | session_2 |
mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select * from table_1 where where id=1 for update; ... 做一些其他处理... | mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql> select * from table_2 where id=1 for update; ... |
select * from table_2 where id =1 for update; 因session_2已取得排他锁,等待 | 做一些其他处理... |
mysql> select * from table_1 where where id=1 for update; 死锁 |
session_1 | session_2 |
mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) | mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) |
mysql> select first_name,last_name from actor where actor_id = 1 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | PENELOPE | GUINESS | +------------+-----------+ 1 row in set (0.00 sec) | |
mysql> insert into country (country_id,country) values(110,'Test'); Query OK, 1 row affected (0.00 sec) | |
mysql> insert into country (country_id,country) values(110,'Test'); 等待 | |
mysql> select first_name,last_name from actor where actor_id = 1 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | PENELOPE | GUINESS | +------------+-----------+ 1 row in set (0.00 sec) | |
mysql> insert into country (country_id,country) values(110,'Test'); ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction |
session_1 | session_2 |
mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) | mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec) |
mysql> select first_name,last_name from actor where actor_id = 1 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | PENELOPE | GUINESS | +------------+-----------+ 1 row in set (0.00 sec) | |
mysql> select first_name,last_name from actor where actor_id = 3 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | ED | CHASE | +------------+-----------+ 1 row in set (0.00 sec) | |
mysql> select first_name,last_name from actor where actor_id = 3 for update; 等待 | |
mysql> select first_name,last_name from actor where actor_id = 1 for update; ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction | |
mysql> select first_name,last_name from actor where actor_id = 3 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | ED | CHASE | +------------+-----------+ 1 row in set (4.71 sec) |
session_1 | session_2 |
mysql> select @@tx_isolation; +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) | mysql> select @@tx_isolation; +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) mysql> set autocommit = 0; Query OK, 0 rows affected (0.00 sec) |
当前session对不存在的记录加for update的锁: mysql> select actor_id,first_name,last_name from actor where actor_id = 201 for update; Empty set (0.00 sec) | |
其他session也可以对不存在的记录加for update的锁: mysql> select actor_id,first_name,last_name from actor where actor_id = 201 for update; Empty set (0.00 sec) | |
因为其他session也对该记录加了锁,所以当前的插入会等待: mysql> insert into actor (actor_id , first_name , last_name) values(201,'Lisa','Tom'); 等待 | |
因为其他session已经对记录进行了更新,这时候再插入记录就会提示死锁并退出: mysql> insert into actor (actor_id, first_name , last_name) values(201,'Lisa','Tom'); ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction | |
由于其他session已经退出,当前session可以获得锁并成功插入记录: mysql> insert into actor (actor_id , first_name , last_name) values(201,'Lisa','Tom'); Query OK, 1 row affected (13.35 sec) |
session_1 | session_2 | session_3 |
mysql> select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql> set autocommit=0; Query OK, 0 rows affected (0.01 sec) | mysql> select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql> set autocommit=0; Query OK, 0 rows affected (0.01 sec) | mysql> select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql> set autocommit=0; Query OK, 0 rows affected (0.01 sec) |
Session_1获得for update的共享锁: mysql> select actor_id, first_name,last_name from actor where actor_id = 201 for update; Empty set (0.00 sec) | 由于记录不存在,session_2也可以获得for update的共享锁: mysql> select actor_id, first_name,last_name from actor where actor_id = 201 for update; Empty set (0.00 sec) | |
Session_1可以成功插入记录: mysql> insert into actor (actor_id,first_name,last_name) values(201,'Lisa','Tom'); Query OK, 1 row affected (0.00 sec) | ||
Session_2插入申请等待获得锁: mysql> insert into actor (actor_id,first_name,last_name) values(201,'Lisa','Tom'); 等待 | ||
Session_1成功提交: mysql> commit; Query OK, 0 rows affected (0.04 sec) | ||
Session_2获得锁,发现插入记录主键重,这个时候抛出了异常,但是并没有释放共享锁: mysql> insert into actor (actor_id,first_name,last_name) values(201,'Lisa','Tom'); ERROR 1062 (23000): Duplicate entry '201' for key 'PRIMARY' | ||
Session_3申请获得共享锁,因为session_2已经锁定该记录,所以session_3需要等待: mysql> select actor_id, first_name,last_name from actor where actor_id = 201 for update; 等待 | ||
这个时候,如果session_2直接对记录进行更新操作,则会抛出死锁的异常: mysql> update actor set last_name='Lan' where actor_id = 201; ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction | ||
Session_2释放锁后,session_3获得锁: mysql> select first_name, last_name from actor where actor_id = 201 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | Lisa | Tom | +------------+-----------+ 1 row in set (31.12 sec) |
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