1. Deadlocks Everything you ever wanted to ask but were too shy
Denis Reznik
Deadlocks Everything you ever wanted to ask but were too shy

2. Sponsors
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3. 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”

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

5. Agenda – More Background
Locks
(S) Shared
(X) Exclusive
(U) Update
(Range*-*) Range
(I*) Intent
(Sch-S) Schema Stability
(Sch-M) Schema Modification
(BU) Bulk Update

6. Agenda – More Background
Transaction Isolation Levels
Pessimistic
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
Optimistic
READ COMMITTED SNAPSHOT
SNAPSHOT

7. Classic Deadlock DEADLOCK! wait wait ID City 1 Kyiv 2 Sofia 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
Sofia 3
Dnipropetrovsk X
wait
wait ID
User
City_Id 1
Dejan Sarka 2
Mihail Mateev 3
Marcin Sheliga 4
John Smith X

8. Demo Common Deadlock

9. Lock Types - Shared X S S

10. Lock Types - Exclusive S X X

11. Demo Common Deadlock

12. Demo When Deadlock Graph does not make sense

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

14. Return different results so they are non-repeatable
NON-REPEATABLE READS
BEGIN TRAN
SELECT * FROM Users
WHERE City = 'Sofia' ID
City 1
Kyiv 2
Sofia 3 4 5
Tokyo 6
New York 7 ID
City 1
Kyiv 2 3
Sofia 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 = 'Sofia'

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

16. Demo When Deadlock Graph does not make sense

17. Demo Object Deadlock

18. 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

19. Lock Types – Intent locksIS IS S

20. Demo Object Deadlock

21. Demo Range Deadlock

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

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

24. Demo Range Deadlock

25. Demo Implicit Deadlock

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

27. Demo Implicit Deadlock

28. Demo NOLOCK Deadlock

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

30. Demo NOLOCK Deadlock

31. Demo Hidden Deadlock

32. Demo One Query Deadlock

33. Demo Undetectable Deadlock

34. 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

35. 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

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37. Thank You!
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