Key Concepts SQL Optimization: Deadlock

: Original Key Concepts SQL Optimization: Deadlock

 

The above mentioned articles transaction, locking, blocking, and finally there is a more extreme case, is the deadlock, which is locked, a situation blocked.

Deadlock is when two transactions were locked resources, but continue to request other resources acquired, it will produce a deadlock.

Deadlock occurs because of:
A, sequential access session to a different table.
B, Session long-running transactions, update in one transaction or a lot of table rows, thus increasing the potential for conflict.
C, Session 1 apply some row locks, session 2 to apply some row locks, then decided to escalate to a table lock.
   If these rows of the same data page, and two sessions to upgrade the lock granularity in the same page, it will deadlocks.

 

1, Session 1

   
   
set lock_timeout 1000
--跟踪死锁--会话1
set transaction isolation level serializable

begin tran

update t
set v = '563'
where idd = 2

waitfor delay '00:00:10'

update t
set v = '963'
where idd = 1

COMMIT
  
  

2, Session 2

   
   
set transaction isolation level serializable

begin tran

update t
set v = '234'
where idd = 1

waitfor delay '00:00:10'

update t
set v = '987'
where idd= 2

commit
  
  

3, then open a conversation, open track

Open trace flag:
              the DBCC TRACEON (the trace # [, ... n-], -. 1) [With NO_INFOMSGS]

Check one or some flag is on or off:
              the DBCC TRACESTATUS (the trace # [, ... n-], -. 1) [With NO_INFOMSGS]

1.trace #: specify one or more of the need to open or to check the status of the trace flags digit
2 -1: If -1 is specified globally open places one or some trace flag
3.with No_InfoMsgs: when this parameter is included in the command, DBCC output messages is prohibited

   
   

--跟踪1222能把详细的死锁信息返回到SQL Server的日志中
--标志位-1表示跟踪标志位1222应该对所有SQL Server连接全局启用
DBCC TraceOn(1222,-1)
go

--验证标志位是否启动
DBCC TraceStatus
go

--关闭标志位
DBCC TraceOff(1222,-1)
go
  
  

4, set the deadlock priority - the possibility of setting deadlock priority, adjust a query session is terminated due to a deadlock running

SET DeadLock_Priority  Low | Normal | High | numeric-priority

   
   
--是当前连接很有可能被终止运行
set deadlock_priority Low

--SQL Server终止回滚代价较小的连接
set deadlock_priority Normal

--减少连接被终止的可能性，除非另一个连接也是High或数值优先级大于5
set deadlock_priority High

--数值优先级：-10到10的值，-10最有可能被终止运行，10最不可能被终止运行，
--两个数字谁大，谁就越不可能在死锁中被终止
set deadlock_priority 10