1. Query Execution Expectation-Reality 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 Chapter)
PASS Regional Mentor, Central and Eastern Europe
Co-author of “SQL Server MVP Deep Dives vol. 2”

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5. Agenda Expectation - Reality 1 Expectation - Reality 2

6. Parameter Sniffing
Expectation – Reality 1

7. Parameter Sniffing - Stored Procedure
Query Processor
EXEC = 1
EXEC = 22
SQL Server Cache
Procedure cache
Procedure cache
Query executes using the query plan created for
@UserId = 1
Plan created and cached for the
@UserId = 1

8. Parameter Sniffing - Parametrized Query
Query Processor
sp_executesql N'SELECT * FROM Users WHERE Id int', 1
SELECT * FROM Users WHERE Id
SELECT * FROM Users WHERE Id
sp_executesql N'SELECT * FROM Users WHERE Id int', 22
SQL Server Cache
Procedure cache
Procedure cache
Plan created and cached for the
@Id = 1
Query executes using the query plan created for
@Id = 1

9. Dynamic SQL – Multiple Plans
Query Processor
SELECT * FROM Users WHERE Id = 1
SELECT * FROM Users WHERE Id = 22
SQL Server Cache
SELECT * FROM Users WHERE Id = 1
Procedure cache
Procedure cache
Procedure cache
Query executed using the query plan, created for the first query.
New query plan created and cached. Query executed using newly created plan.
New query plan again created and cached. Query executed using newly created plan.

10. … … Index Seek SELECT * FROM Users WHERE Id = 523 1 .. 1M 1 .. 2K
2K K
1M-2K .. 1M …
,5K
1,5K K …

11. … … Index Scan SELECT * FROM Users 1 .. 1M 1 .. 2K 2K+1 .. 4K
1M-2K .. 1M …
,5K
1,5K K …

12. DEMO
Parameter Sniffing

13. Cardinality Estimation
Expectation – Reality 2

14. Statistics SELECT * FROM Users WHERE Id BETWEEN 2100 AND 2500
800
2000
2800
4500
5400

15. Out of Range Statistics
SELECT * FROM Users
WHERE Id > 5400 ? 1
800
2000
2800
4500
5400

16. Statistics
SELECT * FROM Users u
INNER JOIN Posts p
ON u.Id = p.OwnerUserId
WHERE u.DisplayName LIKE 'Jeff%'
Users A G L S T
ZZZZ
Posts 1
800
2000
2800
4500
5400

17. Cardinality Estimation
DEMO
Cardinality Estimation

18. Statistics Update SQL Server 2014 and lower default behavior:
20% of rows + 500
SQL Server 2016
Dynamic threshold for tables
with rows
TF 2371 since SQL Server 2008R2 SP1

19. Deadlocks
Expectation – Reality 3

20. Lock Types - Shared X S S

21. Lock Types - Exclusive S X X

22. Lock Types - Update S U U X S U X X U

23. Classic Deadlock DEADLOCK! wait wait ID City 1 Kyiv 2 Kharkiv 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
Kharkiv 3
Dnipropetrovsk X
wait
wait ID
User
City_Id 1
Tony Stark 2
John Smith 3
Clark Kent 4
Ivan Vanko X

24. DEMO
Deadlocks

25. Query Optimization
Expectation – Reality 4

26. Query Processing
Parser
Algebraizer
Optimizer
Executor
Plan Cache

27. Optimization Phase 2 Optimizer Full Set of Optimization Rules
Indexed views
Returns Full Query Plan
All planned checks were done
Good Enough Plan was Found
Timeout
Not Enough Memory

28. DEMO
Query Optimization

29. Summary Parameter Sniffing Cardinality Estimation Deadlocks
Slow in Application. Fast in SSMS.
Cardinality Estimation
SQL Server Version Upgrade
Deadlocks
Non-Detectable Deadlocks
Query Optimization
Query Optimization Timeout

30. Thank You! Denis Reznik Twitter: @denisreznik
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