mysql 開發進階篇系列 12 鎖問題(隔離級別下鎖的差異)
1. innodb在不同隔離級別下的一致性讀及鎖的差異
不同的隔離級別下，innodb處理sql 時采用的一致性讀策略和需要的鎖是不同的，同時，數據恢復和復制機制的特點，也對一些sql的一致性讀策略和鎖策略有很大影響。對於許多sql, 隔離級別越高，innodb給記錄集的鎖就越嚴格(龍其是使用範圍條件的時候)，產生的鎖沖突的可能性也就越高，對並發性事務處理性能的影響也就越大。因此，在應用中，應該盡量使用較低的隔離級別，減少鎖爭用。通常使用Read Commited隔離級別就足夠了， 對於一些確實需要更高隔離級別的事務，可能在程序中執行 SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ 或SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE 動態來改變隔離級別。
下面重點看下REPEATABLE READ與Read commited 鎖申請的不同區別，在增刪改查上申請的鎖都是一致的，但在事務中鎖釋放的時間是不一樣的這點需要註意。
|
SQL |
條件 |
Read uncommited |
Read commited |
Repeatable read |
serializable |
|
Select
|
= |
None locks |
Consisten read/ None locks |
Consisten read/ None locks |
Share locks |
|
範圍 |
None locks |
Consisten read/ None locks |
Consisten read/ None locks |
Share next-key |
|
|
Update |
= |
X(排它鎖) |
X |
X |
X |
|
範圍 |
X next-key |
X next-key |
X next-key |
X next-key |
|
|
Insert |
|
X |
X |
X |
X |
|
REPLACE |
無鍵沖突 |
X |
X |
X |
X |
|
鍵沖突 |
X next-key |
X next-key |
X next-key |
X next-key |
|
|
Delete
|
= |
X |
X |
X |
X |
|
範圍 |
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 |
|
範圍 |
Share locks |
Share locks |
Share next-key |
Share next-key |
|
|
Select ..from For update |
= |
X |
X |
X |
X |
|
範圍 |
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 |
mysql 開發進階篇系列 12 鎖問題(隔離級別下鎖的差異)