Deadlocks strike fear into the heart of even seasoned DBAs-- but they don't have to! In this session, you will get the code to cause a sample deadlock in SQL Server. You'll see how to interpret the deadlock graph to find out where the conflict lies, and how to design an index to make the deadlock disappear. You'll leave the session with the steps you need to confidently tackle future deadlocks.

1. DBA vs. Deadlock

2. Hi, I’m Kendra Little

3. Scary phrases
I’m going to need
you to come in on
Saturday
The code should
work right this time
What backups?
Peer to Peer
Replication
Oops We’re seeing
deadlocks

5. Demo: A deadlock strikes

6. Anatomy of the Deadlock

7. Our frenemy, the deadlock graph

8. Hover to see the inputbuff
UPDATE
Application.StateProvinces …
SELECT …
FROM Application.Cities
JOIN Application.StateProvinces
on …

9. Zooming in on the updates
I have a key lock on the CX of
Application.Countries

10. I want a key lock on the CX of
StateProvinces
I have a key lock on the PK of
Application.Countries

11. Zooming in on the select
I have a Key Lock on the CX
of StateProvinces

12. Secret: it’s not really a U lock
I have a Key Lock on the CX
of StateProvinces

13. I have a Key Lock on the PK
of StateProvinces
I want a Key Lock on the CX
of Countries

14. You’re blocking me
on the CX of
Countries!
You’re blocking me on
the CX of
StateProvinces!

15. The deadlock monitor decided
I WIN!!!!!

16. The deadlock monitor decided
I WIN!!!!!

17. How to solve this conflict

18. Narrow down the conflict
SELECT wants Key
Lock on PK of
Countries
UPDATE wants Key
Lock on PK of
StateProvinces

19. Which indexes will an UPDATE
lock?
SELECT wants Key
Lock on PK of
Countries
UPDATE wants Key
Lock on PK of
StateProvinces

20. Looking into the queries…
I SELECT CountryName and
JOIN on CountryId
I update Countries.
LatestRecordedPopulation


21. Design an index to prevent the deadlock
I SELECT CountryName and
JOIN on CountryId
I update Countries.
LatestRecordedPopulation


22. Let’s create a nonclustered index on
CountryId, INCLUDE CountryName
I could use
that!
LatestRecordedPopulation
won’t be in that index, so I
won’t lock it

23. Demo: We can get along

24. Deadlocks are tough
Deadlock graphs are confusing to read
The “input buffer” won’t have the entirety of multi-
statement transactions
• Often need to dig into the codebase and find
the whole transaction to be able to reproduce
the issue

25. Solving deadlocks with indexes
Make sure a new index is the right move
• Is there already an index leading on
CountryID?
• What is the insert rate and size of the
table?

27. Other solutions
TSQL rewrites
• Changing the queries
• Restructuring transactions
Isolation level changes
• Beware, NOLOCK means “I don’t care if
the data is correct”
• Optimistic locking or snapshot isolation
levels (Test! Test! Test!)