Mysql InnoDB行锁实现方式

 

简述：InnoDB行锁是通过给索引上的索引项加锁来实现的，这一点MySQL与Oracle不同，后者是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！

1.在不通过索引条件查询的时候，InnoDB确实使用的是表锁，而不是行锁。

session_1

session_2

mysql> create table tab_no_index(id int,name varchar(10)) engine=innodb;
Query OK, 0 rows affected (0.15 sec)

mysql> insert into tab_no_index values(1,'1'),(2,'2'),(3,'3'),(4,'4');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

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...

看起来session_1只给一行加了排他锁，但session_2在请求其他行的排他锁时，却出现了锁等待！原因就是在没有索引的情况下，InnoDB只能使用表锁。

 2.表tab_with_index的id字段有索引，name字段没有索引

-- 创建tab_with_index表
mysql> create table tab_with_index(id int,name varchar(10)) engine=innodb;
Query OK, 0 rows affected (0.15 sec)

mysql> alter table tab_with_index add index id(id);
Query OK, 4 rows affected (0.24 sec)
Records: 4  Duplicates: 0  Warnings: 0

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.

 

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。

mysql> alter table tab_with_index drop index name;
Query OK, 4 rows affected (0.22 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> insert into tab_with_index  values(1,'4');
Query OK, 1 row affected (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)

-- 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)
 

mysql> select * from tab_with_index where id = 1 and name = '4' for update;
等待... 

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