The document provides an overview of SQL Server indexing, focusing on clustered and non-clustered indexes. It explains the characteristics, advantages, and scenarios where these indexes are useful or not useful. Key points include the importance of data sort order, index uniqueness, and the impact of indexing on performance.

1. SQL Server
Indexing Basics
August 11, 2015
Mindy Curnutt
Director of DB Architecture
TMW Systems, Inc.
@sqlgirl

2. Clustered Indexes
Nonclustered Indexes
1
2
1

3. Clustered Indexes
• What are they?
• Desired Qualities
2

4. Clustered Indexes
What are they?
• Sort Order of the data within the table
• One per table
3

5. Clustered Indexes
Desired Qualities
• Increasing
• Static
• Narrow Data Type
• Unique
4

6. Clustered Indexes
Increasing
• Data is added to the end of the index
• Reduces fragmentation
5

7. Clustered Indexes
Static
• Changes cause…
• Fragmentation
• Additional overhead for
Non Clustered Indexes
6

8. Clustered Indexes
Narrow Data Type
• Less Data Duplicated
• Smaller…
• Databases
• Backups
• Maintenance Plan Times
• Scans
7

9. Clustered Indexes
Unique
• Not Required
• Prevents Uniqueifier
• Best to be narrow!
8

10. Clustered Indexes
Demos
• No Clustered Index (Heap)
• Non Unique Clustered Index
• Unique Clustered Index
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11. Non Clustered Indexes
• What are they?
• Why are they useful?
• When are they not useful?
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12. Non Clustered Indexes
What are they?
11
Clustered Index Non Clustered Index

13. Non Clustered Indexes
Why are they useful?
• Smaller than table itself
• Seek vs Scan
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SELECT COUNT(*) FROM addressbook
WHERE phone LIKE '707942%'

14. Non Clustered Indexes
When are they not useful?
• When too many rows are returned and all the columns
needed for the query are not present in the nonclustered
index
• Extra work for SQL regarding inserts, updates and
deletes
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15. Non Clustered Indexes
When are they not useful?
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SELECT COUNT(*) FROM addressbook
WHERE phone LIKE '707942%' AND
[street name] LIKE 'Stringer%'