This document provides an overview of SQL Server internals including: - The query processing pipeline including parsing, optimizing, and executing queries. - How the optimizer evaluates and chooses the most efficient query execution plan. - The role of indexes, statistics, and parallelism in query optimization. - Transaction logging and the different SQL Server recovery models. - Key SQL Server memory structures like the buffer pool and query plan cache. - Threading and scheduling within the SQL Server query processor.

1. SQL Server Deep Dive
IT Talk
Denis Reznik
Data Architect at Intapp, Inc.
Microsoft Data Platform MVP

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

3. How We See SQL Server
3 |
Some Magic
Query Result Set

5. SQL Server Network Interface

6. SQL Server Protocols
• Shared Memory
• TCP/IP
• Named Pipes
• VIA (Deprecated)

7. TDS Endpoints
TCP/IP
Shared Memory
Named Pipes
VIA
DAC
TCP/IP
Note: TDS – Tabular Data Stream

8. Connection
• Connection can be Succeed or Not
• https://www.connectionstrings.com/
• Connection string contains settings for Connection Timeout (or Timeout)
• Connection Timeout just limit the time for establishing a connection
• Does not limit the query execution time
• sys.dm_exec_connections
• sys.dm_exec_sessions
• sys.dm_exec_requests
• sys.dm_os_tasks
• … down to the bottom of SQLOS
8 |

9. Query Processor

10. Parser Algebraizer Optimizer Executor
Query Processing
Plan Cache

11. Algebraizing
• Define types for every node in ParseTree
• Check that objects are exist and available in current execution context
• Extract literals from the query
• e.g. WHERE User_Id = 1, 1 is a literal
• Returns BoundTree (QueryTree)
• Logical representation of the query

12. • Goal – get the optimal execution plan
• “Optimal” = Plan with the smallest cost
• Cost = sum_of_all_operators_costs(I/O + CPU +
memory_consumption)
Optimizer
Query Tree
Pre-Optimization
Search for trivial plan
Load statistics (simple)
Simplification – syntax transformation
Optimization

13. Heap
1 .. 100
100 .. 1k
5K .. 6K
1K .. 5K
6K .. 7K
15K .. 21K
12K .. 15K
10K .. 11K
21K .. 22K
22K .. 41K
9K .. 10K
41K .. 51K
7K .. 8K
8K .. 9K
71K .. 1M
51K .. 71K
1M .. 2M
2M .. 3M

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. Index Seek
…
…
1 .. 1M
1 .. 2K 2K+1 .. 4K 1M-2K .. 1M
1 .. 300 301 .. 800 801 .. 1,5K 1,5K+1 .. 2K
SELECT * FROM Users
WHERE Id = 523

16. Index Scan
…
…
1 .. 1M
1 .. 2K 2K+1 .. 4K 1M-2K .. 1M
1 .. 300 301 .. 800 801 .. 1,5K 1,5K+1 .. 2K
SELECT * FROM Users

17. Index Range Scan
…
…
1 .. 1M
1 .. 2K 2K+1 .. 4K 1M-2K .. 1M
1 .. 300 301 .. 800 801 .. 1,5K 1,5K+1 .. 2K
SELECT * FROM Users
WHERE Id BETWEEN 700
AND 1700

18. Non-Clustered Index
…
A .. Z
A .. C C .. K X .. Z
…
1 .. 1M
1 .. 2K 2K+1 .. 4K 1M-2K .. 1M
SELECT * FROM Users
WHERE Name = 'Donald Trump'
1 .. 2K 2K .. 4K 1M-2K .. 1M
Clustered Index
(Id)
Non-Clustered Index
(Name)
Heap

19. Optimizer Phase 0
Load Statistics
Exploration – Plan Alternatives
Estimated cost < 0.2
Return Transaction Processing Plan
Optimization

20. Statistics
500
1000
10
1200
800
1 800 2000 2800 4500 5400
SELECT * FROM Users
WHERE Id BETWEEN 2100 AND 2500
SELECT * FROM Users
WHERE Id BETWEEN 200 AND 5000

21. Query Plan Alternatives SELECT * FROM Users u
INNER JOIN Posts p
ON u.Id = p.OwnerUserId
WHERE u.DisplayName = 'John Snow'
Users
Posts
Posts
Users

22. Query Plan Alternatives
• 1 Table – 1 option
• 2 Tables – 2 options
• 3 Tables – 6 options
• 4 Tables – 24 options
• …
• 10 Tables – 3628800 options
• 1 Table – 1!
• 2 Tables – 2!
• 3 Tables – 3!
• 4 Tables – 4!
• …
• 10 Tables – 10!

23. Optimizer Phase 1
Next set of query rules
Estimate parallel plan (estimation processed twice)
• Max Degree of Parallelism
• Cost Threshold for Parallelism
Estimated cost < 1.0
Returns Quick Plan
Optimization

24. Parallel Query Execution
• Amdal’s Law
Thread 1
Thread 2
Thread 3
Thread 4
1s2s

25. Optimizer Phase 2
Full Set of Optimization Rules
Indexed views
Returns Full Query Plan
• All planned checks were done
• Good Enough Plan was Found
• Timeout
• Memory Limit Exceeded
Optimization

26. Executing
• Execute query using its Query Plan
• Request data from the Storage Engine

27. Parameter Sniffing
Exec SomeProc @p
Optimizer
SQL Server Cache
Procedure cache@p
Plan is cached for the
first value of
Procedure cache

28. DEMO
Parameter Sniffing

29. Storage Engine

30. Buffer Pool
Data Cache
Database File
Access Methods
Buffer Manager
Data Selection
?
What if I load 2GB value from the
disk and read this data using
NOLOCK?

31. Data Modification
Buffer Pool
Data Cache
Database File
Access Methods Buffer Manager
Transaction Log
Log Manager
Log Buffer
?
?
Background
ProcessesLock Manager
? 3 cases

32. Transaction Log
Autogrow
Log Backup or CHECKPOINT

33. FULL Recovery Model
• Log all transactions
• Database can be restored to any point of time
• Highly recommended for critical data
• Transaction truncation occurs after log backup

34. BULK-LOGGED Model
• Log all transactions
• Minimally-Logged operations are logged in another way
• Database can’t be restored to any point of time
• Can be temporary interchanged with FULL recovery model to do
Minimally-Logged operations more effectively
• Transaction truncation occurs after log backup

35. SIMPLE Recovery Model
• Log all transactions like in BULK-LOGGED recovery model
• Transaction truncation occurs after CHECKPOINT

36. DEMO
Log Shrink

37. Lock Types - Shared
S S
X

38. Lock Types - Exclusive
X
X
S

39. Lock Types - Update
U
U
S
SX

40. Lock Types – Intent locks
S
ISIS

41. ID City
1 Kyiv
2 Kharkiv
3 Dnipropetrovsk
BEGIN TRAN
UPDATE Users
SET CityId = 2
WHERE Id = 4
UPDATE City
SET Name = 'Dnipro'
WHERE Id = 3
ID User City_Id
1 Tony Stark 2
2 John Smith 2
3 Clark Kent 2
4 Ivan Vanko 3
Classic Deadlock
X
BEGIN TRAN
UPDATE City
SET Name = 'Dnipro'
WHERE Id = 3
UPDATE Users
SET CityId = 2
WHERE Id = 4
X
DEADLOCK!

42. DEMO
Deadlock

43. SQLOS

44. CPU – Cooperative Multitasking
0
CPUsQueries
1
2
3
Time Quantum Length for each Scheduler is 4 ms

45. Windows Memory Management
0001 0010 0101
0000 1001 0101
0011 1010 1001
1110 0010 0110
0001 0010 0101
0000 1001 0101
0011 1010 1001
Physical Memory (RAM)
Page File
VirtualMemory
Virtual Memory
Manager
SQL Server
Browser
Word

46. Data Cache Query Cache
Locks Memory Memory Grants
Some Small Caches
External Memory
Buffer Pool
SQL Server Memory Consumers

47. Network
Bandwidth
TCP Packages
TCP Packages
TCP Packages
Bandwidth
TCP Packages

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

49. DEMO
Thread Pool Starvation

50. Summary
50 |

51. Thank You!
IT Talk
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