怎么理解Innodb一致性非锁定读

Session A>drop table t;
Session A>create table t(x int primary key);
Session A>insert into t values (1),(5);
Session A>begin;
Session A>select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
Session A>select * from t where x=5 for update;

上面的for update进行的锁定读，此时并没有创建read view注意：for update和lock in share mode都是锁定读，此时并不会创建快照

Session B>begin;
Session B>select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)
Session B>insert into t select 3;
Session B>commit;

Session A>select * from t;  #此时才创建read view 生成快照
+---+
| x |
+---+
| 1 |
| 3 |
| 5 |
+---+

　你可能会发现RR隔离级别，不是应该看不到其他事务修改的数据吗？这正是因为begin开启一个事务，不是begin的时候创建read view，而是第一次进行快照读的时候才创建

ession A>create table a (x int primary key,y int);
Query OK, 0 rows affected (0.17 sec)
#开启一个新事务
Session A>begin;
Query OK, 0 rows affected (0.00 sec)

Session A>select * from a;
Empty set (0.00 sec)
#插入数据
Session A>insert into a select 1,2;
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0
#当前事务能看到insert的数据
Session A>select * from a;
+---+------+
| x | y    |
+---+------+
| 1 |    2 |
+---+------+
1 row in set (0.00 sec)#暂时不commit

#在A提交之前，B开启事务                                      Session B>begin;
Query OK, 0 rows affected (0.00 sec)
#此时查询表a的数据，一致性非锁定读，读到的是事务开启前已经提交的数据，因为B的事务开始的时候A事务还没有提交，故A中insert的数据不会显示
Session B>select * from a;
Empty set (0.00 sec)

#session A提交上面的事务                          Session A>commit;
Query OK, 0 rows affected (0.00 sec)

#在B中select查询，依旧进行的是快照读，故看不到数据                          Seesion B>select * from a;
Empty set (0.00 sec)

Seesion B>select * from aa;
Empty set (0.00 sec)
#INSERT INTO ... SELECT * FROM A;操作
Seesion B>insert into aa select * from a;
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

Seesion B>select * from a;
Empty set (0.00 sec)
#发现直接select表a还是查不到数据，而aa表能看到数据
Seesion B>select * from aa;
+---+------+
| x | y    |
+---+------+
| 1 |    2 |
+---+------+
1 row in set (0.00 sec)

#使用lock in share mode或for update进行当前读能看到表a的数据
Seesion B>select * from a lock in share mode;
+---+------+
| x | y    |
+---+------+
| 1 |    2 |
+---+------+
1 row in set (0.00 sec)

Seesion B>select * from a;
Empty set (0.00 sec)
Seesion B>select * from a for update;
+---+------+
| x | y    |
+---+------+
| 1 |    2 |
+---+------+
1 row in set (0.00 sec)