innodb 的行锁是在有索引的情况下,没有索引的表是锁定全表的.

表锁演示（无索引）

Session1:

mysql> set autocommit=0;

mysql> select * from innodb_test;
+------+-------------+
| id   | name        |
+------+-------------+
|    1 | woshiceshi  | 
|    2 | woshiceshi2 | 
|    3 | woshiceshi3 | 
+------+-------------+

mysql> select * from innodb_test where id = 2 for update;
+------+------------+
| id   | name       |
+------+------------+
|    2 | woshiceshi2 | 
+------+------------+

Session2:

mysql> update innodb_test set name='sjis' where id = 1 ;
处于等待状态....

再回到session1 commit以后，session2就出来结果了（锁定了8秒，过了8秒左右才去session1提交）。

mysql> update innodb_test set name='sjis' where id = 1 ;
Query OK, 1 row affected (8.11 sec)
Rows matched: 1  Changed: 1  Warnings: 0

实验结果是：我在session1的for update 操作看似只锁定ID为2的行其实锁定了全表，以至于后面session2的对ID为1的行update 需要等待Session1锁的释放。

行锁演示（索引为ID）

Session1:
mysql> alter table innodb_test add index idx_id(id);
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from innodb_test where id = 2 for update;
+------+------------+
| id   | name       |
+------+------------+
|    2 | woshiceshi2 | 
+------+------------+

Session2:

mysql> update innodb_test set name='wohaishiceshi' where id = 1 ;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0
mysql> select * from innodb_test where id = 1;           
+------+---------------+
| id   | name          |
+------+---------------+
|    1 | wohaishiceshi | 
+------+---------------+
1 row in set (0.00 sec)

实验结果:这次的锁定是锁定的行，所以没有被锁定的行（ID不为2的行）可以进行update..