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(表格中显示的是申请的锁)。

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