Choosing Your Clustered Index

Jason Strate

e: jstrate@pragmaticworks.com

e: jasonstrate@gmail.com

b: www.jasonstrate.com

t: StrateSQL

Resources jasonstrate.com/go/indexing

Introduction

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Agenda

Introduction Clustered Index or Heap

Clustered Index Qualities

Clustered Index Patterns

Summary

Session Goals

• Define basics related to clustered indexes

• Explain rules behind selecting clustering key

• Demonstrate differences between key column choices

CLUSTERED INDEX OR HEAP

IntroductionClustered Index or

Clustered Index

Qualities

Clustered Index

PatternsSummary

Heap Structure

• Pile of data

– Unordered

– First in, first out

– Scan “happy”

Clustered Index

• Mechanism for storing data

– Ordered

– Structured

– Accessible

– Direct path

Clustered Index

DEMOClustered Index or Heap

Pro Tip: Default to Clustered Indexes, unless heaps are proven to improve performance

CLUSTERED INDEX QUALITIES

Clustered Index

Qualities

Clustered Index

PatternsSummary

Static Narrow

UniqueEver-

increasing

Static Narrow

UniqueEver-

increasing

Static Narrow

UniqueEver-

increasing

Narrow

Rows 4 8 16 32 64

10,000 0.04 0.08 0.15 0.31 0.61

100,000 0.38 0.76 1.53 3.05 6.10

1,000,000 3.81 7.63 15.26 30.52 61.04

10,000,000 38.15 76.29 152.59 305.18 610.35

100,000,000 381.47 762.94 1,525.88 3,051.76 6,103.52

1,000,000,000 3,814.70 7,629.39 15,258.79 30,517.58 61,035.16

Static Narrow

UniqueEver-

increasing

Unique

• Uniqueifier

Rows 4 8 16 32 64

10,000 0.08 0.11 0.19 0.34 0.65

100,000 0.76 1.14 1.91 3.43 6.48

1,000,000 7.63 11.44 19.07 34.33 64.85

10,000,000 76.29 114.44 190.73 343.32 648.50

100,000,000 762.94 1,144.41 1,907.35 3,433.23 6,484.99

1,000,000,000 7,629.39 11,444.09 19,073.49 34,332.28 64,849.85

Static Narrow

UniqueEver-

increasing

Ever-Increasing

1 2 3 4 5 6

Ever-Increasing

1 3 5 6 2 4

Ever-Increasing

1 2 3 4 5 6

These are not unbreakable rules!

CLUSTERED INDEX PATTERNS

Clustered Index

Qualities

Clustered Index

PatternsSummary

Clustered Index Patterns

Identity Column

Surrogate Key

Foreign KeyMulti-

Column

Globally Unique

Identifier

Identity Column

• Non-related value that defines each row uniquely

• Assigned when data is inserted

• Typically uses:

– Int or bigint

– IDENTITY or SEQUENCE

Identity Column

Static

Narrow

Unique

Ever-increasing

DEMOIdentity Column

Surrogate Key

• Unique value is a part of the row

• Assigned in the source data

• Data type dependent on the data

• Value should be unchanging

– Employee ID

– Social Security Number

– Birthdate

Surrogate Key

Static

Narrow

Unique

Ever-increasing

DEMOSurrogate Key

Foreign Key

• One-to-many relationships

• Access path in child via parent

• Piggy-backs identity pattern

– Parent left of child identity value

• Common in many applications

– Order header to details

– Hotels to rooms

Foreign Key

Static

Narrow

Unique

Ever-increasing

DEMOForeign Key

Multi-Column

• Similar to foreign key pattern

• Many-to-many relationships

• Highly dependent on selectivity

• NOT a collection of dimension keys

Multi-Column

Static

Narrow

Unique

Ever-increasing

DEMOMulti-Column

Globally Unique Identifier

• Non-related value that defines each row uniquely

• Assigned when data is inserted

– Or from the application

• Typically uses:

– uniqueidentifier

– NEWID() or NEWSEQUENTIALID()

Globally Unique Identifier

Static

Narrow

Unique

Ever-increasing

DEMOGlobally Unique Identifier

Pro Tip: Clustered indexes should be Static, Narrow, Unique, and Ever-increasing

SUMMARY

Clustered Index

Qualities

Clustered Index

PatternsSummary

Summary

• Clustered indexes are important

• Select clustering keys based on need

• Validate clustered indexes over time

More Information

Expert Performance IndexingFor SQL Server 2012

Jason StrateTed Krueger

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MaintenanceTools

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Name: Jason Strate

Email: jstrate@pragmaticworks.com

Blog: www.jasonstrate.com

Resource: jasonstrate.com/go/indexing

Choosing Your Clustered Index

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