为什么MySQL默认事务断绝级别是RR(2)

root@testdb:3306 12:14:27>truncate table users; Query OK, 0 rows affected (0.08 sec) root@testdb:3306 12:14:29>truncate table class; Query OK, 0 rows affected (0.04 sec) root@testdb:3306 12:14:50>insert into users values(1,'刘备',2,null),(2,'曹操',1,null),(3,'孙 权',3,null),(4,'关羽',2,null),(5,'司马懿',1,null); Query OK, 5 rows affected (0.00 sec) Records: 5 Duplicates: 0 Warnings: 0 root@testdb:3306 12:15:10>insert into class values(1,'魏',null),(2,'蜀',null),(3,'吴',null),(4,'晋',''); Query OK, 4 rows affected (0.00 sec) Records: 4 Duplicates: 0 Warnings: 0