mysql 开发进阶篇系列 12 锁问题(隔离级别下锁的差异)

1. innodb在不同隔离级别下的一致性读及锁的差异
　　不同的隔离级别下，innodb处理sql 时采用的一致性读策略和需要的锁是不同的，同时，数据恢复和复制机制的特点，也对一些sql的一致性读策略和锁策略有很大影响。对于许多sql, 隔离级别越高，innodb给记录集的锁就越严格(龙其是使用范围条件的时候)，产生的锁冲突的可能性也就越高，对并发性事务处理性能的影响也就越大。因此，在应用中，应该尽量使用较低的隔离级别，减少锁争用。通常使用Read Commited隔离级别就足够了， 对于一些确实需要更高隔离级别的事务，可能在程序中执行 SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ 或SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE 动态来改变隔离级别。　

　　下面重点看下REPEATABLE READ与Read commited 锁申请的不同区别，在增删改查上申请的锁都是一致的，但在事务中锁释放的时间是不一样的这点需要注意。

SQL	条件	Read uncommited	Read commited	Repeatable read	serializable
Select	=	None locks	Consisten read/ None locks	Consisten read/ None locks	Share locks
	范围	None locks	Consisten read/ None locks	Consisten read/ None locks	Share next-key
Update	=	X(排它锁)	X	X	X
	范围	X next-key	X next-key	X next-key	X next-key
Insert		X	X	X	X
REPLACE	无键冲突	X	X	X	X
	键冲突	X next-key	X next-key	X next-key	X next-key
Delete	=	X	X	X	X
	范围	X next-key	X next-key	X next-key	X next-key
Select ..from Lock in share mode	=	Share locks	Share locks	Share locks	Share locks
	范围	Share locks	Share locks	Share next-key	Share next-key
Select ..from For update	=	X	X	X	X
	范围	X	Share locks	X next-key	X next-key
Insert into.. Select ..	Innodb_locks_unsafe _for_binlog=off	Share next-key	Share next-key	Share next-key	Share next-key
	Innodb_locks_unsafe _for_binlog=on	None locks	Consisten read/ None locks	Consisten read/ None locks	Share next-key
Create table.. Select ..	Innodb_locks_unsafe _for_binlog=off	Share next-key	Share next-key	Share next-key	Share next-key
	Innodb_locks_unsafe _for_binlog=on	None locks	Consisten read/ None locks	Consisten read/ None locks	Share next-key