固做一下实验，演示下MySQL默认隔离级别REPEATABLE-READ并没有解决幻读问题
幻读演示
MySQL默认隔离级别REPEATABLE-READ（可重复读）
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会话一 |
会话二 |
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MySQL [test]> select * from t1; +------+ | id | +------+ | 1 | | 2 | | 3 | | 4 | +------+ 4 rows in set (0.000 sec) |
MySQL [test]> select * from t1; +------+ | id | +------+ | 1 | | 2 | | 3 | | 4 | +------+ 4 rows in set (0.000 sec) |
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MySQL [test]> begin; Query OK, 0 rows affected (0.000 sec)
注：开启事务一 |
MySQL [test]> begin; Query OK, 0 rows affected (0.000 sec)
注：开启事务二 |
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MySQL [test]> insert into t1 values(5); Query OK, 1 row affected (0.000 sec)
MySQL [test]> select * from t1; +------+ | id | +------+ | 1 | | 2 | | 3 | | 4 | | 5 | +------+ 5 rows in set (0.000 sec)
注：插入一条数据5 |
MySQL [test]> select * from t1; +------+ | id | +------+ | 1 | | 2 | | 3 | | 4 | +------+ 4 rows in set (0.000 sec)
注：因会话一未提交，所以在会话二事务里是看不见更改后的结果的 |
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MySQL [test]> commit; Query OK, 0 rows affected (0.002 sec)
注：会话一执行事务提交 |
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MySQL [test]> select * from t1; +------+ | id | +------+ | 1 | | 2 | | 3 | | 4 | +------+ 4 rows in set (0.000 sec)
MySQL [test]> update t1 set id = id+10; Query OK, 5 rows affected (0.001 sec) Rows matched: 5 Changed: 5 Warnings: 0
注：执行全表更新，id+10 |
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MySQL [test]> select * from t1; +------+ | id | +------+ | 11 | | 12 | | 13 | | 14 | | 15 | +------+ 5 rows in set (0.000 sec)
注：当再次查看时，此时发现有5条数据被更改，产生幻读 |
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MySQL [test]> select version(); +-----------+ | version() | +-----------+ | 8.0.21 | +-----------+ 1 row in set (0.000 sec) |
在MySQL默认隔离级Repeatable Read下，刚才的操作，在会话二未提交的事务里，
会莫名其妙地看到第5条数据，这种现象称为幻读。
固只能通过select ... for update才能避免幻读。
