read view初探

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; Empty set
	insert into tab1 values (1,"1");
select * from tab1; Empty set
	commit;
select * from tab1; Empty set
commit;
select * from tab1; +------+------+ | col1 | col2 | +------+------+ | 1 | 1 | +------+------+

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; +------+------+ | col1 | col2 | +------+------+ | 1 | 1 | +------+------+

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; Empty set (0.00 sec)
	insert into tab1 values (1,"1");
	commit;
select * from tab1; Empty set (0.00 sec)
update tab1 set col2 = 11 where col1 = 1; 1 row affected
select * from tab1; +------+------+ | col1 | col2 | +------+------+ | 1 | 11 | +------+------+

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