A deadlock is a circular blocking chain, where two or more threads are each blocked by the other so that no one can proceed. When the deadlock monitor thread in SQL Server detects a circular blocking chain, it selects one of the participants as a victim, cancels that spid’s current batch, and rolls backs his transaction in order to let the other spids continue with their work. The deadlock victim will get a 1205 error:
Transaction (Process ID 52) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction.
A deadlock is a special type of blocking scenario, but blocking and deadlocking are not the same thing. Sometimes we have people report that they are experiencing "deadlocking" when they are really only seeing blocking.
With very few exceptions, deadlocks are a natural side effect of blocking, not a SQL Server bug. The typical deadlock solution is either a stored proc/app code tweak, or a schema/indexing change.
Here’s how to troubleshoot deadlocks. These steps apply to most deadlocks, and they’ll allow you to resolve many of them without even having to dig into query plans or other nitty gritty details. What’s that? You like digging into query plans, and have nitty grits for breakfast every morning? OK then, we’ll look at a deadlock scenario from the inside out a bit later. But first, here are the basics:
process id=processdceda8 taskpriority=0 logused=0 waitresource=KEY: 2:72057594051493888 (0400a4427a09) waittime=5000 ownerId=24008914 transactionname=SELECT lasttranstarted=2006-09-08T15:54:22.327 XDES=0x8fd9a848 lockMode=S schedulerid=1 kpid=4404 status=suspended spid=54 sbid=0 ecid=0 priority=0 transcount=0 lastbatchstarted=2006-09-08T15:54:22.293 lastbatchcompleted=2006-09-08T15:54:22.293 clientapp=OSQL-32 hostname=BARTD2 hostpid=3408 loginname=bartd isolationlevel=read committed (2) xactid=24008914 currentdb=2 lockTimeout=4294967295 clientoption1=538968096 clientoption2=128056
frame procname=tempdb.dbo.p1 line=2 stmtstart=60 sqlhandle=0x03000200268be70bd
SELECT c2, c3 FROM t1 WHERE c2 = @p1
frame procname=adhoc line=2 stmtstart=32 stmtend=52 sqlhandle=0x020000008a4df52d3
EXEC p1 3
EXEC p1 3
process id=process3c54c58 taskpriority=0 logused=16952 waitresource=KEY: 2:72057594051559424 (0900fefcd2fe) waittime=5000 ownerId=24008903 transactionname=UPDATE lasttranstarted=2006-09-08T15:54:22.327 XDES=0x802ecdd0 lockMode=X schedulerid=2 kpid=4420 status=suspended spid=55 sbid=0 ecid=0 priority=0 transcount=2 lastbatchstarted=2006-09-08T15:54:22.327 lastbatchcompleted=2006-09-08T15:54:22.310 clientapp=OSQL-32 hostname=BARTD2 hostpid=2728 loginname=bartd isolationlevel=read committed (2) xactid=24008903 currentdb=2 lockTimeout=4294967295 clientoption1=538968096 clientoption2=128056
frame procname=tempdb.dbo.p2 line=2 stmtstart=58 sqlhandle=0x030002005fafdb0c
UPDATE t1 SET c1 = FLOOR (c1), c2 = FLOOR (c2) WHERE c1 = @p1
frame procname=adhoc line=2 stmtstart=32 stmtend=52 sqlhandle=0x020000006f878816
EXEC p2 3
EXEC p2 3
keylock hobtid=72057594051559424 dbid=2 objectname=tempdb.dbo.t1 indexname=idx1 id=lock83642a00 mode=S associatedObjectId=72057594051559424
owner id=processdceda8 mode=S
waiter id=process3c54c58 mode=X requestType=wait
keylock hobtid=72057594051493888 dbid=2 objectname=tempdb.dbo.t1 indexname=cidx id=lock83643780 mode=X associatedObjectId=72057594051493888
owner id=process3c54c58 mode=X
waiter id=processdceda8 mode=S requestType=wait
pagelock fileid=1 pageid=95516 dbid=9 objectname="" id=lock177a9e280 mode=IX associatedObjectId=72057596554838016
These are all general recommendations that you can apply to any deadlock without having to really roll up your sleeves and get dirty. If after doing all of this you haven’t resolved it, though, you’ll have to dive a bit deeper and tailor a solution to the specifics of the scenario. Here’s a menu of some common techniques that you can choose from when deciding how best to tackle a deadlock:
1. Begin Transaction
2. Update Part table
2. Update Supplier table
3. Update Supplier table
3. Update Part table
4. Commit Transaction
These two batches may deadlock frequently. If both are about to execute step 3, they may each end up blocked by the other because they both need access to a resource that the other connection locked in step 2.
In a follow-up post I’ll look at a fairly typical deadlock in detail. This will provide an example of what you'd have to do if the 8 high-level steps listed above fail you, forcing you to understand the scenario at a deeper level so that you can craft a custom solution.
(This post series is continued in Deadlock Troubleshooting, Part 2.)
Suba, all deadlocks involve at least two different lock resources, and those resources are often on different indexes. Each of your two sessions holds one of the locks, and is waiting to acquire the lock held by the other session. Take some time to go through the specific steps in this blog post to decode the 1222 output, and hopefully it will give you a clearer understanding of the deadlock scenario.
excellent article and is still useful after all these years.
Thank you for your wonderful article, I used it to diagnose a deadlock.
I have a select statement inside a stored procedure that occasionally causes a deadlock with another stored procedure that is running an update. It appears to be a conflict over an index as you describe in your article.
Unfortunately, for various reasons I cannot alter the indexes on the tables involved in the deadlock.
If I use "SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED" inside the stored procedure that does the read, do I need to call BEGIN TRANSACTION as well? Currently I do not, as the stored procedure does not change any data.
Thanks again for your excellent article.
Awesome article....thanks a ton to author, Bart Duncan...
Loved this article, it made my life easier! Good stuff!