Session 1: Session 2： mysql> show variables like '%tx_isolation%';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.00 sec) Session 1: mysql> select * from t100; --看到的记录为291 mysql> update t100 set free = free -quota where sn=10; | 10 | 300 | 1 | 291 | Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0 mysql> commit;
Query OK, 0 rows affected (0.01 sec) mysql> select * from t100;
+----+------+-------+------+
| sn | id | quota | free |
+----+------+-------+------+
| 1 | 7 | NULL | NULL |
| 2 | 9 | NULL | NULL |
| 3 | 990 | NULL | NULL |
| 4 | 12 | NULL | NULL |
| 5 | 13 | NULL | NULL |
| 6 | 14 | NULL | NULL |
| 7 | 15 | NULL | NULL |
| 8 | 22 | NULL | NULL |
| 9 | 23 | NULL | NULL |
| 10 | 300 | 1 | 291 |
+----+------+-------+------+
10 rows in set (0.00 sec) mysql> update t100 set free = free -quota where sn=10; Session 2看到的仍旧是291
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0 mysql> commit;
Query OK, 0 rows affected (0.01 sec) mysql> select * from t100;
+----+------+-------+------+
| sn | id | quota | free |
+----+------+-------+------+
| 1 | 7 | NULL | NULL |
| 2 | 9 | NULL | NULL |
| 3 | 990 | NULL | NULL |
| 4 | 12 | NULL | NULL |
| 5 | 13 | NULL | NULL |
| 6 | 14 | NULL | NULL |
| 7 | 15 | NULL | NULL |
| 8 | 22 | NULL | NULL |
| 9 | 23 | NULL | NULL |
| 10 | 300 | 1 | 290 |
+----+------+-------+------+ RR模式下 以第一次读为准 All consistent reads within the same transaction read the snapshot established by the first read. 所有的一致性读 在相同事务读取快照 有第一次读创建