如何理解MySQL 5.5 InnoDB表锁

本篇文章为大家展示了如何理解MySQL 5.5 InnoDB表锁，内容简明扼要并且容易理解，绝对能使你眼前一亮，通过这篇文章的详细介绍希望你能有所收获。

对于没有索引的表，MySQL会使用表级锁，写操作不会阻塞读操作，读操作不会阻塞写操作；一个会话的写操作会对整张表加锁，其他会话想修改表需要等到这个会话提交或回滚事务。

会话①
mysql> create table t12(id tinyint(3) unsigned not null,
    ->       name varchar(10) not null)
    ->       engine=innodb auto_increment=8 default charset=gbk;
Query OK, 0 rows affected (0.12 sec)

mysql> show keys from t12;
Empty set (0.00 sec)

mysql> show variables like '%commit%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| autocommit                     | ON    |
| innodb_commit_concurrency      | 0     |
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
3 rows in set (0.00 sec)

mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like '%commit%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| autocommit                     | OFF   |
| innodb_commit_concurrency      | 0     |
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
3 rows in set (0.00 sec)

mysql> insert into t12 values(10,'Neo');
Query OK, 1 row affected (0.00 sec)

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

mysql> select * from t12;
+----+------+
| id | name |
+----+------+
| 10 | Neo  |
+----+------+
1 row in set (0.00 sec)

会话②
mysql> show variables like '%commit%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| autocommit                     | ON    |
| innodb_commit_concurrency      | 0     |
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
3 rows in set (0.00 sec)

mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like '%commit%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| autocommit                     | OFF   |
| innodb_commit_concurrency      | 0     |
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
3 rows in set (0.00 sec)

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

mysql> select * from t12;
+----+------+
| id | name |
+----+------+
| 10 | Neo  |
+----+------+
1 row in set (0.00 sec)

会话①
mysql> update t12 set name='trinity' where id=10;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0

会话②
下面DML语句会一直阻塞
mysql> insert into t12 values(20,'Trinity');

过一段时间会出现超时提示
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

会话①
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)

会话②
mysql> insert into t12 values(20,'Trinity');
Query OK, 1 row affected (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.01 sec)

对于有索引的表，写操作不会阻塞读操作，读操作不会阻塞写操作；如果在MySQL在写操作时使用索引扫描，则会使用行级锁，一个会话的写操作会对修改的行加锁，其他会话想修改这些行需要等到这个会话提交或回滚事务，其他会话对其他行的写操作不受影响，行锁会阻塞表锁；如果MySQL使用全表扫描，则会使用表级锁，一个会话的写操作会对整张表加锁，其他会话想修改表需要等到这个会话提交或回滚事务，表锁会阻塞行锁。

会话①
mysql> create index idx_t12_id on t12(id);
Query OK, 0 rows affected (0.26 sec)
Records: 0  Duplicates: 0  Warnings: 0

查看索引
mysql> show keys from t12;
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name   | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t12   |          1 | idx_t12_id |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

查看执行计划
mysql> explain select * from t12 where id=20;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t12   | ALL  | idx_t12_id    | NULL | NULL    | NULL |    1 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

mysql> delete from t12 where id=20;
Query OK, 2 rows affected (0.00 sec)

mysql> select * from t12;
+----+------+
| id | name |
+----+------+
| 10 | Neo  |
+----+------+
1 row in set (0.00 sec)

会话②

查看执行计划
mysql> explain select * from t12 where id=10;
+----+-------------+-------+------+---------------+------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key        | key_len | ref   | rows | Extra |
+----+-------------+-------+------+---------------+------------+---------+-------+------+-------+
|  1 | SIMPLE      | t12   | ref  | idx_t12_id    | idx_t12_id | 1       | const |    1 |       |
+----+-------------+-------+------+---------------+------------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> update t12 set name='Jack' where id=10;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t12;
+----+---------+
| id | name    |
+----+---------+
| 10 | Jack    |
| 20 | Trinity |
| 20 | Trinity |
+----+---------+
3 rows in set (0.00 sec)

mysql> rollback;
Query OK, 0 rows affected (0.01 sec)

mysql> show processlist;
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
| Id | User            | Host      | db   | Command | Time   | State                                                                       | Info             |
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
|  1 | system user     |           | NULL | Connect | 769140 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL             |
|  2 | system user     |           | NULL | Connect | 769141 | Connecting to master                                                        | NULL             |
| 13 | event_scheduler | localhost | NULL | Daemon  | 621090 | Waiting on empty queue                                                      | NULL             |
| 76 | neo             | localhost | fire | Sleep   |    180 |                                                                             | NULL             |
| 78 | neo             | localhost | fire | Query   |      0 | NULL                                                                        | show processlist |
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
5 rows in set (0.00 sec)

mysql> explain select * from t12;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t12   | ALL  | NULL          | NULL | NULL    | NULL |    1 |       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

下面的更新没有使用索引而使用全表扫描，这样会加表级锁，会处于阻塞状态。
mysql> update t12 set name='Jack';
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

会话①

mysql> rollback;
Query OK, 0 rows affected (0.05 sec)

会话②
随着会话①的回滚操作，会话②执行成功
mysql> update t12 set name='Jack';
Query OK, 3 rows affected (12.41 sec)
Rows matched: 3  Changed: 3  Warnings: 0

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

会话①
mysql> update t12 set name='Jack';
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3  Changed: 3  Warnings: 0

会话②
插入操作会一直处于阻塞状态
mysql> insert into t12 values(30,'Lily');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

会话①
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)

会话②
mysql> insert into t12 values(30,'Lily');
Query OK, 1 row affected (0.09 sec)

两行数据使用了同一个索引，对两个不同的行加锁，也会引起锁等待
mysql> show create table tab_with_index\G
*************************** 1. row ***************************
       Table: tab_with_index
Create Table: CREATE TABLE `tab_with_index` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL,
  KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> select * from tab_with_index where id=1;
+------+------+
| id   | name |
+------+------+
|    1 | 1    |
|    1 | 4    |
+------+------+
2 rows in set (0.00 sec)

mysql> show keys from tab_with_index;
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table          | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tab_with_index |          1 | id       |            1 | id          | A         |           7 |     NULL | NULL   | YES  | BTREE      |         |               |
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

会话①
使用where id=1 and name='1'条件进行查询
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)

会话②
查询where id=1 and name='4'条件进行查询，由于和会话①使用了相同的索引，即使查询了不同的字段，也会引起锁等待
mysql> select * from tab_with_index where id=1 and name='4' for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

在这种情况下，可以考虑创建联合索引
会话①
mysql> create index idx_id_name on tab_with_index(id,name);
Query OK, 0 rows affected (0.24 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> select * from tab_with_index where id=1 and name='4' for update;
+------+------+
| id   | name |
+------+------+
|    1 | 4    |
+------+------+
1 row in set (0.00 sec)

会话②
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)

InnoDB存储引擎的表使用不同索引的阻塞例子

会话①
mysql> show create table tab_with_index\G
*************************** 1. row ***************************
       Table: tab_with_index
Create Table: CREATE TABLE `tab_with_index` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL,
  KEY `id` (`id`),
  KEY `idx_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> desc tab_with_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | MUL | NULL    |       |
| name  | varchar(10) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> select * from tab_with_index;
+------+------+
| id   | name |
+------+------+
|    1 | 1    |
|    2 | 2    |
|    3 | 3    |
|    4 | 4    |
|    1 | 4    |
+------+------+
5 rows in set (0.00 sec)

mysql> explain select * from tab_with_index where id=1 for update;
+----+-------------+----------------+------+---------------+------+---------+-------+------+-------+
| id | select_type | table          | type | possible_keys | key  | key_len | ref   | rows | Extra |
+----+-------------+----------------+------+---------------+------+---------+-------+------+-------+
|  1 | SIMPLE      | tab_with_index | ref  | id            | id   | 5       | const |    2 | NULL  |
+----+-------------+----------------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> select * from tab_with_index where id=1;
+------+------+
| id   | name |
+------+------+
|    1 | 1    |
|    1 | 4    |
+------+------+
2 rows in set (0.00 sec)

mysql> select * from tab_with_index where id=1 for update;
+------+------+
| id   | name |
+------+------+
|    1 | 1    |
|    1 | 4    |
+------+------+
2 rows in set (0.01 sec)

会话②
mysql> explain select * from tab_with_index where name='4' for update;
+----+-------------+----------------+------+---------------+----------+---------+-------+------+-----------------------+
| id | select_type | table          | type | possible_keys | key      | key_len | ref   | rows | Extra                 |
+----+-------------+----------------+------+---------------+----------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | tab_with_index | ref  | idx_name      | idx_name | 13      | const |    2 | Using index condition |
+----+-------------+----------------+------+---------------+----------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

mysql> select * from tab_with_index where name='4' for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

上述内容就是如何理解MySQL 5.5 InnoDB表锁，你们学到知识或技能了吗？如果还想学到更多技能或者丰富自己的知识储备，欢迎关注亿速云行业资讯频道。