CODEWITHSUNDEEP
sqlsql-serversql-server-2008deadlock
CraftyFella
CraftyFella

Reputation: 7568

Why would a select/read cause an exclusive (X) lock?

Some deadlocks are occurring in a database used by an API I'm working on. This is only happening when the API is load tested. During the initial investigation and research some importing indexes seemed to be missing and have now been applied. This seems to have resolved the deadlock issue. Before the indexes were applied SQL was performing an index scan. After the indexes were applied SQL was performing an index seek.

The reason for this question is to solidify my understanding of deadlocks. I'm still a bit confused as to why without the indexes the select statements caused an exclusive (X) Lock in MS SQL?

This is purely, I think, down to me not understanding the deadlock graph. From what I can see the 2 processes in the picture below are both doing selects.. so how can that cause the exclusive (X) lock? Is there something not on the graph perhaps?

Here is the deadlock graph which occurs without the extra indexes:

enter image description here

...and here is the XML ( from that graph:

<deadlock-list>
 <deadlock victim="process4c3708">
  <process-list>
   <process id="process4c3708" taskpriority="0" logused="1580" waitresource="KEY: 5:72057594038910976 (a94bedf44228)" waittime="99" ownerId="13602992" transactionname="user_transaction" lasttranstarted="2012-10-03T10:59:34.830" XDES="0x8cbf23b0" lockMode="S" schedulerid="3" kpid="7588" status="suspended" spid="67" sbid="0" ecid="0" priority="0" trancount="1" lastbatchstarted="2012-10-03T10:59:35.027" lastbatchcompleted="2012-10-03T10:59:35.020" clientapp=".Net SqlClient Data Provider" hostname="DEVMACHINE" hostpid="8440" loginname="user" isolationlevel="read committed (2)" xactid="13602992" currentdb="5" lockTimeout="4294967295" clientoption1="671088672" clientoption2="128056">
    <executionStack>
     <frame procname="adhoc" line="1" stmtstart="170" stmtend="728" sqlhandle="0x02000000b3a88339052734f326b9dbb95deb1d46fe4d192d">
select basefolder1_.Identifier as col_0_0_ from TradeInfo tradepr0_ inner join BaseFolder basefolder1_ on tradepr0_.BaseFolderId=basefolder1_.Id where basefolder1_.BoxId=@p0 and tradepr0_.PersonId=@p1 and tradepr0_.CurrentTrade=1 and tradepr0_.IsActive=1;     </frame>
     <frame procname="unknown" line="1" sqlhandle="0x000000000000000000000000000000000000000000000000">
unknown     </frame>
    </executionStack>
    <inputbuf>
(@p0 uniqueidentifier,@p1 uniqueidentifier,@p2 uniqueidentifier,@p3 uniqueidentifier)select basefolder1_.Identifier as col_0_0_ from TradeInfo tradepr0_ inner join BaseFolder basefolder1_ on tradepr0_.BaseFolderId=basefolder1_.Id where basefolder1_.BoxId=@p0 and tradepr0_.PersonId=@p1 and tradepr0_.CurrentTrade=1 and tradepr0_.IsActive=1;
select basefolder1_.Identifier as col_0_0_ from TradeInfo tradepr0_ inner join BaseFolder basefolder1_ on tradepr0_.BaseFolderId=basefolder1_.Id where basefolder1_.BoxId=@p2 and tradepr0_.PersonId=@p3 and tradepr0_.SuspendedTrade=1 and tradepr0_.IsSuspended=1;
    </inputbuf>
   </process>
   <process id="process5dd4c8" taskpriority="0" logused="1580" waitresource="KEY: 5:72057594038910976 (b34811986aff)" waittime="301" ownerId="13602927" transactionname="user_transaction" lasttranstarted="2012-10-03T10:59:34.660" XDES="0xafb9f950" lockMode="S" schedulerid="4" kpid="2076" status="suspended" spid="61" sbid="0" ecid="0" priority="0" trancount="1" lastbatchstarted="2012-10-03T10:59:35.020" lastbatchcompleted="2012-10-03T10:59:35.020" clientapp=".Net SqlClient Data Provider" hostname="DEVMACHINE" hostpid="8440" loginname="user" isolationlevel="read committed (2)" xactid="13602927" currentdb="5" lockTimeout="4294967295" clientoption1="671088672" clientoption2="128056">
    <executionStack>
     <frame procname="adhoc" line="1" stmtstart="170" stmtend="728" sqlhandle="0x02000000b3a88339052734f326b9dbb95deb1d46fe4d192d">
select basefolder1_.Identifier as col_0_0_ from TradeInfo tradepr0_ inner join BaseFolder basefolder1_ on tradepr0_.BaseFolderId=basefolder1_.Id where basefolder1_.BoxId=@p0 and tradepr0_.PersonId=@p1 and tradepr0_.CurrentTrade=1 and tradepr0_.IsActive=1;     </frame>
     <frame procname="unknown" line="1" sqlhandle="0x000000000000000000000000000000000000000000000000">
unknown     </frame>
    </executionStack>
    <inputbuf>
(@p0 uniqueidentifier,@p1 uniqueidentifier,@p2 uniqueidentifier,@p3 uniqueidentifier)select basefolder1_.Identifier as col_0_0_ from TradeInfo tradepr0_ inner join BaseFolder basefolder1_ on tradepr0_.BaseFolderId=basefolder1_.Id where basefolder1_.BoxId=@p0 and tradepr0_.PersonId=@p1 and tradepr0_.CurrentTrade=1 and tradepr0_.IsActive=1;
select basefolder1_.Identifier as col_0_0_ from TradeInfo tradepr0_ inner join BaseFolder basefolder1_ on tradepr0_.BaseFolderId=basefolder1_.Id where basefolder1_.BoxId=@p2 and tradepr0_.PersonId=@p3 and tradepr0_.SuspendedTrade=1 and tradepr0_.IsSuspended=1;
    </inputbuf>
   </process>
  </process-list>
  <resource-list>
   <keylock hobtid="72057594038910976" dbid="5" objectname="ccp.dbo.TradeInfo" indexname="PK__Activity__3214EC07060DEAE8" id="lock8011dd00" mode="X" associatedObjectId="72057594038910976">
    <owner-list>
     <owner id="process5dd4c8" mode="X"/>
    </owner-list>
    <waiter-list>
     <waiter id="process4c3708" mode="S" requestType="wait"/>
    </waiter-list>
   </keylock>
   <keylock hobtid="72057594038910976" dbid="5" objectname="ccp.dbo.TradeInfo" indexname="PK__Activity__3214EC07060DEAE8" id="lock8a864e00" mode="X" associatedObjectId="72057594038910976">
    <owner-list>
     <owner id="process4c3708" mode="X"/>
    </owner-list>
    <waiter-list>
     <waiter id="process5dd4c8" mode="S" requestType="wait"/>
    </waiter-list>
   </keylock>
  </resource-list>
 </deadlock>
</deadlock-list>

Upvotes: 0

Views: 2152

Answers (2)

Remus Rusanu
Remus Rusanu

Reputation: 294267

None of the SELECT's in the deadlock XML actually acquires an X lock. The two resources involved in the deadlock are currently owned in X mode and requested in S mode. Which implies that each transaction has previously locked the resource (in this case, a key), perhaps it had run an DML statement that updated/inserted that row. The SELECT statements only want to read the row, so they request S mode.

Upvotes: 2

Martin Smith
Martin Smith

Reputation: 453243

It wouldn't (without locking hints)

Presumably there was a preceding statement in the same transaction (not shown in the deadlock graph) that actually acquired the X lock.

Notice that logused="1580" which also wouldn't happen if the statement was a standalone SELECT

Upvotes: 3

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