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。