在Repeatable Read隔离级别下,一个事务可能会遇到幻读(Phantom Read)的问题。

幻读是指,在一个事务中,第一次查询某条记录,发现没有,但是,当试图更新这条不存在的记录时,竟然能成功,并且,再次读取同一条记录,它就神奇地出现了。大理石构件如何选择

我们仍然先准备好students表的数据:

mysql> select * from students;
+----+-------+
| id | name  |
+----+-------+
|  1 | Alice |
+----+-------+
1 row in set (0.00 sec)

然后,分别开启两个MySQL客户端连接,按顺序依次执行事务A和事务B:

1SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
2BEGIN;BEGIN;
3 SELECT * FROM students WHERE id = 99;
4INSERT INTO students (id, name) VALUES (99, 'Bob'); 
5COMMIT; 
6 SELECT * FROM students WHERE id = 99;
7 UPDATE students SET name = 'Alice' WHERE id = 99;
8 SELECT * FROM students WHERE id = 99;
9 COMMIT;
01-03 05:44