mysql RC下不存在则插入

mysql版本：5.7

目的：在RC下，name列上仅有key索引，并发插入name时不出现重复数据

RC不加gap lock，并且复合select语句是不加锁的快照读，导致两个事务同时进行都可插入，测试如下：

client1：

mysql> set tx_isolation='read-committed';

mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set, 1 warning (0.00 sec)

mysql> create table t (id int primary key, name int, key(name))engine=innodb;
Query OK, 0 rows affected (0.24 sec)

.......

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
+----+------+
7 rows in set (0.00 sec)

mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set (0.03 sec)

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

mysql> insert into t select 8,8 from dual where not exists (select name from t where name=8);
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
|  8 |    8 |
+----+------+
8 rows in set (0.00 sec)

client2设置同client1，设置略，然后起事务插入：

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

mysql> insert into t select 9,8 from dual where not exists (select name from t where name=8);
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
|  9 |    8 |
+----+------+
8 rows in set (0.00 sec)

可以看到并未阻塞，这不同于RR，在RR下会阻塞，因为加了gap lock。

难道这时候没有加任何锁吗，其实并不是，client1执行如下，并查看锁：

mysql> select name from t where name=8 lock in share mode;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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 |
+-----------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+
| 164163:469:4:10 | 164163      | S         | RECORD    | `test1`.`t` | name       |        469 |         4 |       10 | 8, 9      |
| 164168:469:4:10 | 164168      | X         | RECORD    | `test1`.`t` | name       |        469 |         4 |       10 | 8, 9      |
+-----------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.00 sec)

看看加锁的数据。client2如下：

mysql> select name from t where name=8 lock in share mode;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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 |
+----------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+
| 164168:469:4:9 | 164168      | S         | RECORD    | `test1`.`t` | name       |        469 |         4 |        9 | 8, 8      |
| 164163:469:4:9 | 164163      | X         | RECORD    | `test1`.`t` | name       |        469 |         4 |        9 | 8, 8      |
+----------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.00 sec)

看看加锁的数据，可见client1和2都上了锁，是在insert时上的。

那么为了能达到加锁阻塞的目的，可以使用如下方式，client1：

mysql> insert into t select 8,8 from dual where not exists (select name from t where name=8 for update);
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

client2则阻塞：

mysql> insert into t select 9,8 from dual where not exists (select name from t where name=8 for update);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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 |
+-----------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+
| 164170:469:4:10 | 164170      | X         | RECORD    | `test1`.`t` | name       |        469 |         4 |       10 | 8, 8      |
| 164169:469:4:10 | 164169      | X         | RECORD    | `test1`.`t` | name       |        469 |         4 |       10 | 8, 8      |
+-----------------+-------------+-----------+-----------+-------------+------------+------------+-----------+----------+-----------+
2 rows in set, 1 warning (0.00 sec)

如果client1在client2阻塞时 commit：

mysql> insert into t select 8,8 from dual where not exists (select name from t where name=8 for update);
Query OK, 1 rows affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> commit;
Query OK, 0 rows affected (0.04 sec)

client2：

mysql> insert into t select 9,8 from dual where not exists (select name from t where name=8 for update);
Query OK, 0 rows affected (4.79 sec)
Records: 0  Duplicates: 0  Warnings: 0

还有一个需要注意的地方是，如果不加for update，则并发插入时，都会插入新数据，client1：

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

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
|  8 |    8 |
+----+------+
8 rows in set (0.00 sec)

mysql> insert into t select 9,9 from dual where not exists (select name from t where name=9);
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
|  8 |    8 |
|  9 |    9 |
+----+------+
9 rows in set (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.03 sec)

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
|  8 |    8 |
|  9 |    9 |
| 10 |    9 |
+----+------+
10 rows in set (0.00 sec)

与client1并发执行的client2：

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

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
|  8 |    8 |
+----+------+
8 rows in set (0.00 sec)

mysql> insert into t select 10,9 from dual where not exists (select name from t where name=9);
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
|  8 |    8 |
| 10 |    9 |
+----+------+
9 rows in set (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.04 sec)

mysql> select * from t;
+----+------+
| id | name |
+----+------+
|  1 |    1 |
|  2 |    2 |
|  3 |    3 |
|  4 |    4 |
|  5 |    5 |
|  6 |    6 |
|  7 |    7 |
|  8 |    8 |
|  9 |    9 |
| 10 |    9 |
+----+------+
10 rows in set (0.00 sec)

可见，根本起不到不存在则插入的效果。