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
