主要看并发事务中不存在则插入（只有key索引）的阻塞情况。

表定义：

mysql> desc user;
+-------------+------------------+------+-----+-------------------+----------------+
| Field       | Type             | Null | Key | Default           | Extra          |
+-------------+------------------+------+-----+-------------------+----------------+
| id          | int(10) unsigned | NO   | PRI | NULL              | auto_increment |
| name        | varchar(50)      | NO   | MUL | NULL              |                |
| password    | char(20)         | NO   |     | NULL              |                |
| regist_time | timestamp        | NO   |     | CURRENT_TIMESTAMP |                |
+-------------+------------------+------+-----+-------------------+----------------+
4 rows in set (0.00 sec)

事务隔离级别：RR

mysql版本：5.7

client1：

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

mysql> select * from user;
+----+------+----------+---------------------+
| id | name | password | regist_time         |
+----+------+----------+---------------------+
|  1 | a    | a        | 2018-03-11 16:32:43 |
|  2 | b    | b        | 2018-03-11 16:33:09 |
|  3 | c    | c        | 2018-03-11 16:33:39 |
+----+------+----------+---------------------+
3 rows in set (0.00 sec)

mysql> insert into user(name,password) select 'd','d' from dual where not exist (select name from user where name='d');
Query OK, 1 row affected (0.00 sec)Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from user;
+----+------+----------+---------------------+
| id | name | password | regist_time         |
+----+------+----------+---------------------+
|  1 | a    | a        | 2018-03-11 16:32:43 |
|  2 | b    | b        | 2018-03-11 16:33:09 |
|  3 | c    | c        | 2018-03-11 16:33:39 |
|  4 | d    | d        | 2018-03-11 17:03:35 |
+----+------+----------+---------------------+
4 rows in set (0.00 sec)

然后启动client2：

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

mysql> select * from user;
+----+------+----------+---------------------+
| id | name | password | regist_time         |
+----+------+----------+---------------------+
|  1 | a    | a        | 2018-03-11 16:32:43 |
|  2 | b    | b        | 2018-03-11 16:33:09 |
|  3 | c    | c        | 2018-03-11 16:33:39 |
+----+------+----------+---------------------+
3 rows in set (0.00 sec)

mysql> select * from user where name='d';
Empty set (0.02 sec)

mysql> insert into user (name,password) select 'd','d' from dual where not exists (select name from user where name='d');

client2 执行“ insert into user (name,password) select 'd','d' from dual where not exists (select name from user where name='d'); ”出现阻塞，直到超时或client1 commit。

client2 直接执行插入操作则不会阻塞：

mysql> insert into user(name, password) values ('d','d');
Query OK, 1 row affected (0.00 sec)

client2 执行：

mysql> insert into user (name,password) select 'e','e' from dual where not exists (select name from user where name='e');

也会出现阻塞。但是执行：

mysql> insert into user (name,password) select '12','12' from dual where not exists (select name from user where name='12');
Query OK, 1 row affected (0.02 sec)
Records: 1  Duplicates: 0  Warnings: 0

并不会阻塞。

另：如果已经存在name='d'的数据，client1执行"insert not exists"后并不会插入也不会加锁，client2执行时也不会阻塞。

查看锁（client2 插入'd'时的情况）：

mysql> select * from information_schema.innodb_locks;
+-------------------------+-----------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
| lock_id                 | lock_trx_id     | lock_mode | lock_type | lock_table     | lock_index | lock_space | lock_page | lock_rec | lock_data |
+-------------------------+-----------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
| 422016582501824:462:4:8 | 422016582501824 | S         | RECORD    | `test1`.`user` | name       |        462 |         4 |        8 | 'd', 11   |
| 162094:462:4:8          | 162094          | X         | RECORD    | `test1`.`user` | name       |        462 |         4 |        8 | 'd', 11   |
+-------------------------+-----------------+-----------+-----------+----------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.02 sec)

client2 当插入'z'时也会阻塞，但lock_data还会是：

mysql> select * from information_schema.innodb_locks;
+----------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+------------------------+
| lock_id        | lock_trx_id | lock_mode | lock_type | lock_table     | lock_index | lock_space | lock_page | lock_rec | lock_data              |
+----------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+------------------------+
| 162131:462:4:1 | 162131      | X         | RECORD    | `test1`.`user` | name       |        462 |         4 |        1 | supremum pseudo-record |
| 162094:462:4:1 | 162094      | S         | RECORD    | `test1`.`user` | name       |        462 |         4 |        1 | supremum pseudo-record |
+----------------+-------------+-----------+-----------+----------------+------------+------------+-----------+----------+------------------------+
2 rows in set, 1 warning (0.00 sec)

也就是'z'是加锁的上界，插入'x'和'~'也是这种情况。

之所以'12'不会锁，'d'和其以后的都会锁，是因为mysql为了防止幻读，还锁住了下一行，因为最大的是'd'，所以锁住区域为（'d', +∞），另一个区域是（'c', 'd'）。如果插入的不是这个区域的都不会阻塞。

RC和RR加锁区别请见：RR和RC复合语句加锁

当client2 插入'A'、'B'时居然不阻塞也插入不了：

mysql> insert into user (name,password) select 'A','A' from dual where not exists (select name from user where name='A');
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into user (name,password) select 'B','B' from dual where not exists (select name from user where name='B');
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

client1也插入不了'A'，只有直接执行时才可以：

mysql> insert into user (name,password) select 'A','A' from dual where not exists (select name from user where name='A');
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into user(name,password) values('A','A');
Query OK, 1 row affected (0.00 sec)

之所以出现无法插入'A'、'B'，是因为不区分大小写，测试一下便知：

mysql> select * from user where name='a';
+----+------+----------+---------------------+
| id | name | password | regist_time         |
+----+------+----------+---------------------+
|  1 | a    | a        | 2018-03-11 16:32:43 |
| 44 | A    | A        | 2018-03-11 20:56:42 |
+----+------+----------+---------------------+
2 rows in set (0.00 sec)

要想区分大小写，建表时需要相应设置，也可以在查询时使用：

mysql> select * from user where binary name='a';
+----+------+----------+---------------------+
| id | name | password | regist_time         |
+----+------+----------+---------------------+
|  1 | a    | a        | 2018-03-11 16:32:43 |
+----+------+----------+---------------------+
1 row in set (0.01 sec)

另：on duplicate key只适用于unique key，如果不是unique，总是会插入

mysql> insert into user(name,password) values('d','d') on duplicate key update password='e';

这时会插入一条name='d',password='d'的记录。