1. Everything you ever wanted to ask but were too shy
Deadlocks
Everything you ever wanted to ask but were too shy
Denis Reznik
Data Architect at Intapp, Inc.
Microsoft Data Platform MVP

2. About me Denis Reznik Kyiv, Ukraine Data Architect at Intapp, Inc.
Microsoft Data Platform MVP
Co-Founder of Ukrainian Data Community Kyiv
PASS Regional Mentor, CEE
Co-author of “SQL Server MVP Deep Dives 2”

3. Agenda Classic Deadlock Common Deadlock
When Deadlock Graph doesn’t Make Sense
Object Deadlock
Range Deadlock
Implicit Deadlock
Hidden Deadlock
NOLOCK Deadlock
One Query Deadlock
Undetectable Deadlock

4. Classic Deadlock DEADLOCK! wait wait ID City 1 Kyiv 2 Ljubljana 3
BEGIN TRAN
UPDATE Users
SET CityId = 2
WHERE Id = 4
UPDATE City
SET Name = 'Dnipro'
WHERE Id = 3
BEGIN TRAN
UPDATE City
SET Name = 'Dnipro'
WHERE Id = 3
UPDATE Users
SET CityId = 2
WHERE Id = 4 ID
City 1
Kyiv 2
Ljubljana 3
Dnipropetrovsk X
wait
wait ID
User
City_Id 1
Dejan Sarka 2
Mladen Prajdic 3
Matija Lah 4
John Smith X

5. How to Deal with the Deadlock?
Catch the Deadlock
Find the Root Cause
Fix the Deadlock

6. Demo
Common Deadlock

7. Lock Types - Shared X S S

8. Lock Types - Exclusive S X X

9. Deadlocks. When Graph does not make sense.
Demo
Deadlocks. When Graph does not make sense.

10. Lock Types - Update s U U X S U X X U

11. Return different results so they are non-repeatable
NON-REPEATABLE READS
BEGIN TRAN
SELECT * FROM Users
WHERE City = 'Ljubljana' ID
City 1
Kyiv 2
Ljubljana 3 4 5
Tokyo 6
New York 7 ID
City 1
Kyiv 2 3
Ljubljana 4 5
Tokyo 6
New York 7
UPDATE Users
SET City = 'Kyiv'
WHERE Id = 2 X S
Return different results so they are non-repeatable S
... S
SELECT * FROM Users
WHERE City = 'Ljubljana'

12. REPEATABLE READ ID City 1 Kyiv 2 Ljubljana 3 4 5 Tokyo 6 New York 7 ID
BEGIN TRAN
SELECT * FROM Users
WHERE City = 'Ljubljana' ID
City 1
Kyiv 2
Ljubljana 3 4 5
Tokyo 6
New York 7 ID
City 1
Kyiv 2 3
Ljubljana 4 5
Tokyo 6
New York 7
UPDATE Users
SET City = 'Kyiv'
WHERE Id = 2 X S S
... S
SELECT * FROM Users
WHERE City = 'Ljubljana'
COMMIT

13. Demo
Object Deadlock

14. Lock Escalation … >= 25% 1 2 Table S S Buffer Pool n = 5000
Memory for locks 1 S
>= 25% 2 S … 3 S n S
n = 5000
if lock can’t be put on table, process will try this every n rows

15. Lock Types – Intent locksIS IS S

16. Demo
Range Deadlock

17. PHANTOM RECORDS Phantom Record ID City 1 Kyiv 2 Ljubljana 3 4 5 Tokyo
BEGIN TRAN
SELECT * FROM Users
WHERE City = 'Ljubljana' ID
City 1
Kyiv 2
Ljubljana 3 4 5
Tokyo 6
New York 7 ID
City 1
Kyiv 2
Ljubljana 3 4 8 5
Tokyo 6
New York 7
INSERT INTO Users
VALUES (8, 'Ljubljana') S S
... S S
SELECT * FROM Users
WHERE City = 'Ljubljana'
Phantom
Record

18. SERIALIZABLE ID City 1 Kyiv 2 Ljubljana 3 4 8 5 Tokyo 6 New York 7 ID
BEGIN TRAN
SELECT * FROM Users
WHERE City = 'Ljubljana' ID
City 1
Kyiv 2
Ljubljana 3 4 8 5
Tokyo 6
New York 7 ID
City 1
Kyiv 2
Ljubljana 3 4 5
Tokyo 6
New York 7
INSERT INTO Users
VALUES (8, 'Ljubljana')
RANGE S-S
...
SELECT * FROM Users
WHERE City = 'Ljubljana'
COMMIT

19. Demo
Range Deadlock

20. READ COMMITTED SNAPSHOT
SELECT * FROM Users
WHERE City = 'Kyiv'
BEGIN TRAN
UPDATE Users
SET City = 'Ljubljana'
WHERE City = 'Kyiv' ID
City 1
Ljubljana 2 3 4 5 6
New York 7 ID
City 1
Kyiv 2
Ljubljana 3 4 5 6
New York 7 X
SELECT * FROM Users
WHERE City = 'Kyiv'
Version Store ID
City 1
Kyiv
tempdb

21. SNAPSHOT ID City 1 Ljubljana 2 3 4 5 6 New York 7 ID City 1 Kyiv 2
BEGIN TRAN
UPDATE Users
SET City = 'Ljubljana'
WHERE City = 'Kyiv'
BEGIN TRAN
UPDATE Users
SET City = 'London'
WHERE City = 'Kyiv' X
COMMIT
Version Store ID
City 1
Kyiv
tempdb

22. Demo
Implicit Deadlock

23. READ UNCOMMITTED (NOLOCK)
SELECT * FROM Users
WHERE City = 'Kyiv' ID
City 1
Kyiv 2
Ljubljana 3 4 5 6 7 ID
City 1
Kyiv 2
Ljubljana 3 4 5 6
New York 7
BEGIN TRAN
UPDATE Users
SET City = 'Kyiv'
WHERE City = 'New York'
SELECT * FROM Users
WHERE City = 'Kyiv'
ROLLBACK
Dirty Read X X
SELECT * FROM Users
WHERE City = 'Kyiv'

24. Demo
NOLOCK Deadlock

25. Three Rules of Deadlocks
If deadlock is possible, it will occur.
Deadlock should not be solved, before the root cause of it wasn’t found.
There are no unsolvable deadlocks. But there can be solutions, which will not suit you completely
25 |

26. How to Avoid?
Design database so, that it will be no possibility for a deadlock occur
Ha-ha-ha 
Modify tables in the same order
Choose appropriate Transaction Isolation Level
Consider optimistic ones
Keep transactions as small as possible
Use stress-testing for your application or database
26 |

27. Thank You! Denis Reznik Twitter: @denisreznik
Blog:
Facebook:
LinkedIn: