SQL Server允许X锁定共享锁，这是导致死锁的原因

Question

这是一个奇怪的发现。我正在一个会话上进行选择和删除，并在同一行的另一个会话中删除，请参阅下面的代码。 departmentid有聚集索引。

------Session 1----------
set transaction isolation level repeatable read
begin transaction
select * from HumanResources.Department

------session 2 ------------
set transaction isolation level repeatable read
begin transaction
delete from HumanResources.Department
where departmentid = 9

--session 1 -----------

delete from HumanResources.Department
where departmentid = 9

这是应该发生的事情。

• 会话1获取每行的共享锁
• 现在并发会话2执行delete语句并且应该对该行设置[u]锁，然后尝试将u锁定转换为X锁定，因为会议1
持有s锁
• 现在，会话1执行delete语句尝试将X锁定，但被会话2的u锁定阻止。因此发生死锁。

然而，当我看到死锁图时，我得到了不同的图片。死锁图表示会话1授予S锁定。第2节授予X锁（我在这里有困惑）如何在S锁上授予x锁。请参阅下面的死锁图：

<victimProcess id="process5d1a718"/>
  <process-list>
   <process id="process5d1a718" taskpriority="0" logused="0" waitresource="KEY: 7:72057594073907200 (15ac4ff5e281)" waittime="2723" ownerId="21790" transactionname="user_transaction" lasttranstarted="2016-03-06T15:44:12.863" XDES="0x6056c08" lockMode="X" schedulerid="2" kpid="6016" status="suspended" spid="52" sbid="0" ecid="0" priority="0" trancount="2" lastbatchstarted="2016-03-06T15:44:53.147" lastbatchcompleted="2016-03-06T15:44:12.863" clientapp="Microsoft SQL Server Management Studio" hostname="SAMAR-PC" hostpid="3620" loginname="samar-PC\samar" isolationlevel="repeatable read (3)" xactid="21790" currentdb="7" lockTimeout="4294967295" clientoption1="671090784" clientoption2="390200">
    <executionStack>
     <frame procname="" line="1" stmtstart="24" sqlhandle="0x0200000033392c164aa2164aaf9c828974edf396604cba95">
     </frame>
     <frame procname="" line="1" sqlhandle="0x02000000e46e292caa2d578b8876190ff31f36bfdf42b892">
     </frame>
    </executionStack>
    <inputbuf>
delete from HumanResources.Department
where departmentid = 9    </inputbuf>
   </process>
   <process id="process19b1c8" taskpriority="0" logused="0" waitresource="KEY: 7:72057594074103808 (10aa4711661d)" waittime="21212" ownerId="21981" transactionname="user_transaction" lasttranstarted="2016-03-06T15:44:34.657" XDES="0x6b4c280" lockMode="X" schedulerid="1" kpid="4148" status="suspended" spid="55" sbid="0" ecid="0" priority="0" trancount="2" lastbatchstarted="2016-03-06T15:44:34.657" lastbatchcompleted="2016-03-06T15:43:36.970" lastattention="2016-03-06T15:33:58.423" clientapp="Microsoft SQL Server Management Studio - Query" hostname="SAMAR-PC" hostpid="3620" loginname="samar-PC\samar" isolationlevel="repeatable read (3)" xactid="21981" currentdb="7" lockTimeout="4294967295" clientoption1="671090784" clientoption2="390200">
    <executionStack>
     <frame procname="" line="3" stmtstart="24" sqlhandle="0x0200000033392c164aa2164aaf9c828974edf396604cba95">
     </frame>
     <frame procname="" line="3" stmtstart="136" sqlhandle="0x02000000c3907d12b9a0c511ab9b3c2966aa00fd5258d812">
     </frame>
    </executionStack>
    <inputbuf>
set transaction isolation level repeatable read
begin transaction
delete from HumanResources.Department
where departmentid = 9    </inputbuf>
   </process>
  </process-list>
  <resource-list>
   <keylock hobtid="72057594073907200" dbid="7" objectname="" indexname="" id="locka1d5040" mode="X" associatedObjectId="72057594073907200">
    <owner-list>
     <owner id="process19b1c8" mode="X"/>
    </owner-list>
    <waiter-list>
     <waiter id="process5d1a718" mode="X" requestType="wait"/>
    </waiter-list>
   </keylock>
   <keylock hobtid="72057594074103808" dbid="7" objectname="" indexname="" id="locka1d4780" mode="S" associatedObjectId="72057594074103808">
    <owner-list>
     <owner id="process5d1a718" mode="S"/>
    </owner-list>
    <waiter-list>
     <waiter id="process19b1c8" mode="X" requestType="wait"/>
    </waiter-list>
   </keylock>
  </resource-list>
 </deadlock>
</deadlock-list>
</value>
      <text />
    </data>
  </event>
</RingBufferTarget>

我有另一个发现，这也很奇怪。如果我更改了会话1的select语句，那就是我在其中添加where子句，我没有遇到任何死锁。我只看到会话2被阻止，会话1用departmentid = 9删除该行。请参阅下面的查询： -

------Session 1----------
set transaction isolation level repeatable read
begin transaction
select * from HumanResources.Department
where departmentid = 9

------session 2 ------------
set transaction isolation level repeatable read
begin transaction
delete from HumanResources.Department
where departmentid = 9

--session 1 -----------

delete from HumanResources.Department
where departmentid = 9

有人可以在这里分享一些亮点。我的所有概念似乎都破灭了。

Answer 1

该表有departmentid，name，groupname，modifieddate。

Departmentid具有聚簇索引，并且名称具有非聚簇索引和其他列 - groupname和modifieddate包含在列中。发生了什么。

1. 在会话1中选择查询将s锁定在非聚集索引上。

2. 会话2将x锁定置于聚集索引上的departmentid = 9。

3. 现在更改聚簇索引将更新非聚簇索引，因此会话2尝试将X锁定置于非聚簇索引，但被会话1的S锁定阻止。

4. 会话1尝试通过在聚集索引上放置由会话2的X锁定阻止的x锁来删除同一行。因此陷入僵局。

我通过在会话1中使用select语句（index = Clusteredindexname）提示来解决它：

------Session 1----------
set transaction isolation level repeatable read
begin transaction
select * from HumanResources.Department with (index = Clusteredindexname)