The document outlines 5 "detective stories" or troubleshooting scenarios involving SQL Server performance issues. Story 1 discusses slow query performance due to locking and blocking. Story 2 involves a missing database. Story 3 is about a database file that won't shrink due to fragmentation. Story 4 looks at high CPU with no queries running. And story 5 examines parameter sniffing issues causing intermittent performance. The presenter provides demonstrations of resolving each issue.

1. True SQL Server Detective
Denis Reznik
Intapp

2. About Me
• Denis Reznik
• Kyiv, Ukraine
• Data Architect at Intapp
• 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”

3. Agenda
• Detective Story 1
• Why my query is slow?
• Detective Story 2
• Where is my database, dude?
• Detective Story 3
• File size is not reducing.
• Detective Story 4
• My CPU is burning but no queries are executing!
• Detective Story 5 (?)
• How deep is the rabbit hole?

4. Detective Story 1
Why my query is slow?

5. Case Details
• User Experience
• Everything is slow
• System is not working
• Technical Details
• Query Timeout errors
• Connection Timeout Errors
• Long-Running Queries

6. Query Execution
Thread Pool
Running
Suspended
Runnable
Scheduler
Logical CPU
Worker (Thread)
Connection

7. LCK_M_* Waits
0
SchedulersRunnable Queue
1
2 1
Suspended Queue
2
SELECT * FROM Users
WHERE Name = 'Scott Nall'
UPDATE Users
SET Name = 'Optimus Prime'
WHERE Id = 3
Id Name
1 John Snow
2 Peter Partner
3 Scott Nall
4 Jane Dow
1
Users
2
3
2
Id Name
1 John Snow
2 Peter Partner
3 Optimus Prime
4 Jane Dow
LCK_M_X
LCK_M_X
2
X S

8. Demo
Why my query is slow?

9. Detective Story 2
Where is my database?

10. Case Details
• User Experience
• Database disappeared
• System is not working
• Technical Details
• No database on the server

11. Demo
Where is my database?

12. Detective Story 3
File size is not reducing

13. Case Details
• User Experience
• Disk space is running out
• Database file is not shrinking
• Preparing for disaster
• Technical Details
• Database Shrink operation stuck even on 1 MB Shrink
• Database is in FULL recovery mode
• Database is a part of availability group
• Database indexes are highly fragmented (50%-90%)

14. Clustered Index
…
…
1 .. 1M
1 .. 2K 2K+1 .. 4K 1M-2K .. 1M
1 .. 300 301 .. 800 801 .. 1,5K 1,5K+1 .. 2K

15. Demo
File size is not reducing

16. Allocation Units
UPDATE
IN_ROW_DATA (HOBT) ROW_OVERFLOW_DATA
(SLOB)

17. Allocation Units
IN_ROW_DATA (HOBT) LOB_DATA (BLOB)

18. Detective Story 4
My CPU is burning but no queries are executing!

19. Case Details
• User Experience
• No explanation for the consistent 10%-20% of CPU utilization
• During the peak load this additional utilization become critical
• Technical Details
• Let’s check them on Demo

20. Demo
My CPU is burning but no queries are executing!

21. Detective Story 5
How deep is the rabbit hole?

22. Case Details
• User Experience
• Application become slow apparently
• In a minute the same operation can be fast again
• Technical Details
• Query execution is really slow
• Running the same query in SSMS is almost instantaneous

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

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

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

26. Demo
How deep is the rabbit hole?

27. Summary
• Detective Story 1
• Why my query is slow?
• Detective Story 2
• Where is my database?
• Detective Story 3
• File size is not reducing.
• Detective Story 4
• My CPU is burning but no queries are executing!
• Detective Story 5 (?)
• How deep is the rabbit hole?

28. Thank You!
Denis Reznik
Twitter: @denisreznik
Email: denisreznik@gmail.com
Blog: http://reznik.uneta.com.ua
Facebook: https://www.facebook.com/denis.reznik.5
LinkedIn: http://ua.linkedin.com/pub/denis-reznik/3/502/234