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 |
原文地址：https://www.cnblogs.com/MrHSR/p/9403316.html
时间： 2024-11-05 14:50:40