innodb为实现MVCC所使用的内部快照，RR(REPEATABLE READ)隔离级别下在第一次查询时创建read view,RC(READ COMMITTED)隔离级别下会在每次查询时创建read view
以下测试在RR隔离级别下，数据库版本为5.7.20
1.
| session A | session B |
| start transaction; | |
| start transaction; | |
|
select * from tab1; |
|
| insert into tab1 values (1,"1"); | |
|
select * from tab1; |
|
| commit; | |
|
select * from tab1; |
|
|
commit; |
|
|
select * from tab1; |
|
结论：在已经查询后，其他事务做的修改，在本事务不可见
2.
| session A | session B |
| truncate table tab1; | |
|
start transaction; |
|
| start transaction; | |
| insert into tab1 values (1,"1"); | |
| commit; | |
|
select * from tab1; |
结论：尽管事务A比事务B先开始，但是第一次查询在B事务提交后，所以可以查询到结果
3.
| session A | session B |
| truncate table tab1; | |
|
start transaction; |
|
| start transaction; | |
|
select * from tab1; |
|
| insert into tab1 values (1,"1"); | |
| commit; | |
|
select * from tab1; |
|
|
update tab1 set col2 = 11 where col1 = 1; 1 row affected |
|
|
select * from tab1; |
结论：虽然事务A看不到事务B做的修改，但是修改也会影响事务B已经提交的数据，且修改发生后，下次查询会重新刷新read view
参考网址：
1. https://dev.mysql.com/doc/refman/5.6/en/innodb-consistent-read.html
2. http://kabike.iteye.com/blog/1820553
原文：https://www.cnblogs.com/wangzheng1024/p/9403361.html