从wait_type入手模拟SQL Server Lock

一、LCK_M_S，等待获取共享锁

开始一SQL TRAN，其中执行对某数据的UPDATE。但并不COMMIT，也不ROLLBACK。

begin tranupdate [dbo].[HR_Employee] set [Description]='ZZ'

这样，便使用排它锁锁定了该[Employee]表。

在另一会话中，执行对该表的SELECT操作。至此，死锁产生。

select * from [dbo].[HR_Employee]

使用下列script查询当前锁情况。

 1 SELECT wt.blocking_session_id                    AS BlockingSessesionId 2         ,sp.PRogram_name                           AS ProgramName 3         ,COALESCE(sp.LOGINAME, sp.nt_username)     AS HostName     4         ,ec1.client_net_address                    AS ClientIpAddress 5         ,db.name                                   AS DatabaseName         6         ,wt.wait_type                              AS WaitType                     7         ,ec1.connect_time                          AS BlockingStartTime 8         ,wt.WAIT_DURATION_MS/1000                  AS WaitDuration 9         ,ec1.session_id                            AS BlockedSessionId10         ,h1.TEXT                                   AS BlockedSQLText11         ,h2.TEXT                                   AS BlockingSQLText12   FROM sys.dm_tran_locks AS tl13   INNER JOIN sys.databases db14     ON db.database_id = tl.resource_database_id15   INNER JOIN sys.dm_os_waiting_tasks AS wt16     ON tl.lock_owner_address = wt.resource_address17   INNER JOIN sys.dm_exec_connections ec118     ON ec1.session_id = tl.request_session_id19   INNER JOIN sys.dm_exec_connections ec220     ON ec2.session_id = wt.blocking_session_id21   LEFT OUTER JOIN master.dbo.sysprocesses sp22     ON SP.spid = wt.blocking_session_id23   CROSS APPLY sys.dm_exec_sql_text(ec1.most_recent_sql_handle) AS h124   CROSS APPLY sys.dm_exec_sql_text(ec2.most_recent_sql_handle) AS h2

发现该LOCK的wait_type为LCK_M_S，意味着后一会话在等待着获取对该表的共享锁已完成查询工作。

二、LCK_M_U，等待获取更新锁。

发起一SQL会话，在其中使用更新锁（UPDLOCK）SELECT数据，而后WAIT一定的时间。

1 begin tran2 select * from [dbo].[HR_Employee] WITH (UPDLOCK) where [Id]=73 waitfor delay '00:01:00' 4 update [dbo].[HR_Employee] set [Description]='ZZ' where [Id]=75 commit tran

在wait的时间内，[Id]=7的行被更新锁锁住。

发起另一会话，使用更新锁（UPDLOCK）完成SELECT操作。

1 select * from [dbo].[HR_Employee] WITH (UPDLOCK)

发现后一会话被block。wait_type为LCK_M_U，表示其在等待该表的更新锁。

三、LCK_M_X，等待获取排它锁

将上一小节中第二个会话的操作改为UPDATE。

update [dbo].[HR_Employee] set [Description]='ZZy' where [Id]=7

后一会话同样被block，但这次的wait_type为LCK_M_X，表明其在等待用于UPDATE DATA的排它锁。