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查询表级锁争用情况
show status like ‘table%’;
MySQL表级锁的锁模式
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)
|
如何加表锁
在用LOCK TABLES给表显式加表锁时,必须同时取得所有涉及到表的锁,并且MySQL不支持锁升级。也就是说,在执行LOCK TABLES后,只能访问显式加锁的这些表,不能访问未加锁的表;同时,如果加的是读锁,那么只能执行查询操作,而不能执行更新操作。其实,
在自动加锁的情况下也基本如此,MyISAM总是一次获得SQL语句所需要的全部锁。这也正是MyISAM表不会出现死锁(Deadlock Free)的原因。
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
|
并发插入(Concurrent Inserts)
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
|
通过定期在系统空闲时段执行 OPTIMIZE TABLE语句来整理空间碎片,收回因删除记录而产生的中间空洞。有关OPTIMIZE TABLE语句的详细介绍,可以参见第18章中“两个简单实用的优化方法”一节的内容。
MyISAM的锁调度
写进程先获得锁。不仅如此,即使读请求先到锁等待队列,写请求后到,写锁也会插到读锁请求之前!这是因为MySQL认为写请求一般比读请求要重要。
这也正是MyISAM表不太适合于有大量更新操作和查询操作应用的原因,因为,大量的更新操作会造成查询操作很难获得读锁,从而可能永远阻塞。这种情况有时可能会变得非常糟糕!幸好我们可以通过一些设置来调节MyISAM 的调度行为。
max_write_lock_count设置一个合适的值,当一个表的读锁达到这个值后,MySQL就暂时将写请求的优先级降低,给读进程一定获得锁的机会。
这里还要强调一点:一些需要长时间运行的查询操作,也会使写进程“饿死”!因此,应用中应尽量避免出现长时间运行的查询操作,不要总想用一条SELECT语句来解决问题,因为这种看似巧妙的SQL语句,往往比较复杂,执行时间较长,在可能的情况下可以通过使用中间表等措施对SQL语句做一定的“分解”,使每一步查询都能在较短时间完成,从而减少锁冲突。如果复杂查询不可避免,应尽量安排在数据库空闲时段执行,比如一些定期统计可以安排在夜间执行。
背景知识
1.事务(Transaction)及其ACID属性
2.并发事务处理带来的问题
3.事务隔离级别
,“更新丢失”通常是应该完全避免的。但防止更新丢失,并不能单靠数据库事务控制器来解决,需要应用程序对要更新的数据加必要的锁来解决,因此,防止更新丢失应该是应用的责任。
多版本并发控制(MultiVersion Concurrency Control,简称MVCC或MCC),也经常称为多版本数据库。
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
|
获取InnoDB行锁争用情况
show status like ‘innodb_row_lock%’;
InnoDB的行锁模式及加锁方法
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
|
InnoDB存储引擎的共享锁例子
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
|
InnoDB存储引擎的排他锁例子
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)
|
InnoDB行锁实现方式
通过在数据块中对相应数据行加锁来实现的。InnoDB这种行锁实现特点意味着:只有通过索引条件检索数据,InnoDB才使用行级锁,否则,InnoDB将使用表锁!
InnoDB存储引擎的表在不使用索引时使用表锁例子
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
|
InnoDB存储引擎的表在使用索引时使用行锁例子
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)
|
由于MySQL的行锁是针对索引加的锁,不是针对记录加的锁,所以虽然是访问不同行的记录,但是如果是使用相同的索引键,是会出现锁冲突的。应用设计的时候要注意这一点。
InnoDB存储引擎使用相同索引键的阻塞例子
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
|
InnoDB存储引擎的表使用不同索引的阻塞例子
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;
|
因此,在分析锁冲突时,别忘了检查SQL的执行计划,以确认是否真正使用了索引。关于MySQL在什么情况下不使用索引的详细讨论,参见本章“索引问题”一节的介绍。
计划,我们可以清楚地看到了这一点。
间隙锁(Next-Key锁)
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)
|
恢复和复制的需要,对InnoDB锁机制的影响
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)
|
InnoDB在不同隔离级别下的一致性读及锁的差异
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
|
什么时候使用表锁
关于死锁
InnoDB存储引擎中的死锁例子
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)
|
InnoDB存储引擎中隔离级别引起的死锁例子1
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)
|
当隔离级别为READ COMMITTED时,如果两个线程都先执行SELECT…FOR UPDATE,判断是否存在符合条件的记录,如果没有,就插入记录。此时,只有一个线程能插入成功,另一个线程会出现锁等待,当第1个线程提交后,第2个线程会因主键重出错,但虽然这个线程出错了,却会获得一个排他锁!这时如果有第3个线程又来申请排他锁,也会出现死锁。
InnoDB存储引擎中隔离级别引起的死锁例子2
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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