The document discusses common myths about SQL Server indexing, clarifying misconceptions such as primary keys always being clustered and the importance of index column order. It emphasizes the role of clustered indexes in physically ordering data, the impact of fill factor on insert operations, and the nuances between clustered vs. heap indexes. Overall, it aims to improve understanding of SQL indexing to enhance database performance.

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5 SQL Server
Indexing Myths

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Introduction
Jason
Strate
e: jstrate@pragmaticworks.com
e: jasonstrate@gmail.com
b: www.jasonstrate.com
t: StrateSQL
Resources www.jasonstrate.com/go/indexing

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Unicorns aren’t real!

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I want to
believe!

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But husky
rhinos are!

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Primary Keys Are Always Clustered

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Primary Keys Are Always Clustered
Primary keys are clustered by default
Common point of confusion
Often the desired behavior
Is the primary key a good clustered index?
Static and stable
Ever increasing

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Primary Keys Are Always Clustered
Demo

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Index Column Order Doesn’t Matter

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Index Column Order Doesn’t Matter
Using and seeking an index are not the same
Read execution plans carefully
Validate scans versus seeks
Indexes are sorted left to right
All columns are not required
Scans aren’t necessarily bad

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Index Column Order Doesn’t Matter
Col1 Col2
A 1
A 2
A 3
B 1
B 2
B 3
C 1
C 2
C 3
A 1

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Index Column Order Doesn’t Matter
Demo

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Clustered Indexes Are Physically Ordered

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Clustered Indexes Are Physically Ordered
Clustered indexes sort data within the index
Accurately find specific values
A before B, B before C
Physical sorted at the tree level
Path data access directly to the required page
Logically sorted at the page level
Minimize page rewrites on DML

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Clustered Indexes Are Physically Ordered
Demo

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Fill Factor Affects Insert Operations

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Fill Factor Affects Insert Operations
Fill factor leaves space on data pages
Why do we do this?
Leave space for DML operations
Mitigate page splits during DML operations
Provides a buffer for data variability

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Fill Factor Affects Insert Operations

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Fill Factor Affects Insert Operations

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Fill Factor Affects Insert Operations
Demo

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Every Table Should Have a Heap/Clustered Index

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Every Table Should Have a XXX Index
Blanket statements suck
Doesn’t account for all situations
Or even most situations
Not 50/50
Not 90/10
Is there a safe answer?

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Heap / Clustered Indexes
Heap
• Non-selective access
• Bulk Loaded tables
• Fragmentation prone
with high inserts
• Staging tables
Clustered Index
• Highly modified data
• Singleton access
• Range scans
• Key ordered results

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5 SQL Server
Indexing Myths
Questions?

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Name: Jason Strate
Email: jstrate@pragmaticworks.com
Blog: www.jasonstrate.com
Resource: www.jasonstrate.com/go/indexing
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