三种级别

record lock  锁住某一行记录  

gap lock     锁住某一段范围中的记录 

next key lock 是前两者效果的叠加
实验环境
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)
mysql> select * from t;
+------+
| id   |
+------+
|    2 |
|    8 |
|   19 |
|   22 |
+------+
mysql> create index t_ind on t(id);
Query OK, 0 rows affected (8.52 sec)
 同时打开两个会话，并打开事物
会话一
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> delete from t where id=19;
Query OK, 1 row affected (0.00 sec)
会话二
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into t values(55);
Query OK, 1 row affected (0.00 sec)
mysql> insert into t values(10);
^C^C -- query aborted
ERROR 1317 (70100): Query execution was interrupted
mysql> 
mysql> insert into t values(1);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t values(12);
^C^C -- query aborted
ERROR 1317 (70100): Query execution was interrupted
mysql> 
会话二说明8-22之间被锁住了，无法插入数据.这就是gap lock                                                      

取消gap lock
修改事物的隔离级别到READ COMMITTED