mysql下事务和锁

 

 

	事务一	事务二
	start transaction;
		start transaction;
P1	select c from a where a=1;
		update a set c=c+1 where a=1;
P2	select c from a where a=1 for update;
Q1		select c from a where a=1;
		commit;
P3	select c from a where a=1;
	commit;

 

同一个事务中

select c from a where a=1 for update

与

select c from a where a=1

取出的值不一样

 

 

REPEATABLE READ （可重复读）	P1=P3=0 P2=Q1=1
READ COMMITTED	P1=0 P2=P3=Q1=1

 

 

 

	事务一	事务二
	start transaction;
P1	select c from a where a=1;
		start transaction;
		update a set c=c+1 where a=1;
Q1		select c from a where a=1;
P2	select c from a where a=1;
		commit;
P3	select c from a where a=1;

 

 一致性的非锁定读：

通过Undo段实现，不同的隔离级别读取的快照数据不同

多版本并发控制（Multi Version Concurrency Control, MVCC)

 

REPEATABLE READ （可重复读）	P1=P2=P3=0 Q1=1
READ COMMITTED	P1=P2=0 P3=Q1=1

 

 

	事务一	事务二	结果（更新后 读最新值）
	start transaction;
		start transaction;
Q1		select c from a where a=1;	0
		update a set c=c+1 where a=1;
Q1		select c from a where a=1;	1
		commit;
P1	select c from a where a=1;		1

	事务一	事务二	结果（更新后 读最新值）
	start transaction;
		start transaction;
P1	select c from a where a=1;		0
		update a set c=c+1 where a=1;
Q1		select c from a where a=1;	1
	update a set c=c+1 where a=1;
		commit;
P2	select c from a where a=1;		3

 

 

	事务一	事务二
	begin;
P1	update a set a=2 where a=1;
		begin;
Q1		update a set c=37 where a=1;
	commit;

 

 

 写一致性

REPEATABLE READ （可重复读）	重新启动更新
READ COMMITTED	重新启动更新

 

事务一	事务二	next-key-lock
begin;
select c from a where a<3 lock in share mode;
	begin;
	Insert into a(a) select 1;	锁等待
Commit;

SELECT@@tx_isolation;

 

SET [SESSION |GLOBAL] TRANSACTION ISOLATION LEVEL {READ UNCOMMITTED | READ COMMITTED |REPEATABLE READ | SERIALIZABLE}

SET  SESSION TRANSACTION ISOLATION LEVELREPEATABLE READ;