.NET Fest 2019. Денис Резник. Когда SQL Server Best Practices не работают

Тема доклада
KYIV 2019
When SQL Server Best Practices do not work
.NET CONFERENCE #1 IN UKRAINE

.NET LEVEL UP
About Me
.NET CONFERENCE #1 IN UKRAINE KYIV 2019
• 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”

.NET LEVEL UP
Agenda
• Database Should Be Normalized
• Each Table Should Have a Clustered Index
• Column Datatypes Should Be as Small as Possible
• Cost Threshold for Parallelism Should be Higher Than Default
• Max Degree of Parallelism Should Be Set to Best Practices Value
• Bonus: NOLOCK Side Effect

KYIV 2019
Database Should Be Normalized

.NET LEVEL UP
First Normal Form (1NF)
Company User Phone Phone Type
Microsoft John Dow +380969785732 NULL
Microsoft John Dow +32345409123 NULL
Microsoft Larry McGregor +45678904692 NULL
Oracle Corp. John Snow +380988958371 NULL
Amazon Jack Snack +23348902385 Home
Amazon Jack Snack +69058763287 Work
Each cell contains an atomic value
Company User Phone
Microsoft John Dow Tel1: +380969785732, Tel2: +32345409123
Microsoft Larry McGregor Tel: +45678904692
Oracle Corp. John Snow +380988958371
Amazon Jack Snack Home: +23348902385 Work: +69058763287
UsersUsers

KYIV 2019
Demo

KYIV 2019
Conclusion
• Normalize when in makes sense
• Denormalize when it makes sense
• Do not know what to do? Normalize.
• Do not afraid of Denormalization.

KYIV 2019
Each Table Should Have a Clustered Index

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

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

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

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 = 'John Dow'
1 .. 2K 2K .. 4K 1M-2K .. 1M
Clustered Index
(Id)
Non-Clustered Index
(Name)
Heap

KYIV 2019
Demo

KYIV 2019
Conclusion
• Do not afraid Heaps
• RID Lookup requires less IO than Key Lookup
• Consider Heaps for:
• High Key Lookup workloads
• Big tables with high insert rate

KYIV 2019
Column Datatypes Should Be as Small as Possible

.NET LEVEL UP
Possible Size Reduction
• int (4 Bytes) -> smallint (2B) -> tinyint (1B) -> bit (up to 1 Byte)
• float -> decimal
• datetime -> smalldatetime(?) -> datetime2
• nchar (2 Bytes per character) -> char (1 Byte per character)
• nvarchar (2 Bytes per character) -> varchar (1 Byte per character)

KYIV 2019
Demo

.NET LEVEL UP
Data Type Precedence
1. user-defined data types (highest)
2. sql_variant
3. xml
4. datetimeoffset
5. datetime2
6. datetime
7. smalldatetime
8. date
9. time
10. float
11. real
12. decimal
13. money
14. smallmoney
15. bigint
16. int
17. smallint
18. tinyint
19. bit

KYIV 2019
Conclusion
• Works for almost all datatypes
• Use correct datatypes for each column
• Prefer (by default) nvarchar over varchar

KYIV 2019
Cost Threshold for Parallelism Should be Higher Than Default

• 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

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

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

Query Plan Alternatives
• 1 Table – 1 option
• 2 Tables – 2 options
• …
• 1 Table – 1!
• 2 Tables – 2!
• 3 Tables – 3!
• 4 Tables – 4!
• …
• 10 Tables – 10!

Dynamic Programming
• JOIN(A,B,C,D)
• JOIN(A,B,D,C)
• JOIN(A,C,D,B)
• JOIN(A,D,B,C)
• JOIN(A,D,C,B)
• JOIN(B,A,C,D)
• JOIN(B,A,D,C)
• …
• O(N!)
• JOIN(A,B,C,D)
• A – Optimal Access Path
• B – Optimal Access Path
• … pruning
• (A,B) – Optimal Access Path
• … pruning
• (A,B),(C)
• … pruning
• O(𝑁2 𝑛−1
)

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

KYIV 2019
Demo

KYIV 2019
Conclusion
• Good to set it higher on a fresh server
• Bad to set in higher for the working system
• Setting it for a working system requires validation
• Validate it for top 5-20 CPU intensive queries

KYIV 2019
Max Degree of Parallelism Should Be Set to Best Practices Value

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

Optimizer Phase 2 (Last Phase)
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
Optimization

KYIV 2019
Demo

KYIV 2019
Conclusion
• High DOP usually force query to waste CPU
• But not in every situation
• MAXDOP 1 gives the Optimizer a bit more time

KYIV 2019
Bonus: NOLOCK Side Effect

KYIV 2019
Bonus: NOLOCK Side Effect
Demo

Extent N
Allocation Order Scan
…
1 .. 1M
1 .. 2K 2K+1 .. 4K 1M-2K .. 1M
Clustered Index (Id)
Extent 1
Index Allocation Map (IAM)
Extent 1
Extent 2
Extent N

.NET LEVEL UP
Summary
• Database Should Be Normalized
• Each Table Should Have a Clustered Index
• Column Datatypes Should Be as Small as Possible
• Cost Threshold for Parallelism Should be Higher Than Default
• Max Degree of Parallelism Should Be Set to Best Practices Value
• Add “If it does make sense” after each of the statements above
• Bonus: NOLOCK and Cursor Threshold

KYIV 2019
When SQL Server Best Practices do not work
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

.NET Fest 2019. Денис Резник. Когда SQL Server Best Practices не работают

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.NET Fest 2019. Денис Резник. Когда SQL Server Best Practices не работают