introduction to clustered indexes and heaps

Introduction to Clustered Indexes and Heaps

Jason Strate

e: [email protected]

e: [email protected]

b: www.jasonstrate.com

t: StrateSQL

Resources jasonstrate.com/go/indexing

Introduction

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Heaps

Clustered

Agenda

Introduction The Heap Clustered Index

Index Patterns Summary


Session Goals

• Define differences between heaps and clustered indexes

• Explain design considerations

• Demonstrate differences between key column choices


THE HEAP

Introduction The HeapClustered

IndexIndex

PatternsSummary


Heap Structure

• Pile of data

– Unordered

– First in, first out

– Scan “happy”

• Table Scan Access

• Forwarded Records


Heap


THE HEAP

Demo


CLUSTERED INDEX


IndexIndex

PatternsSummary


Clustered Index

• Mechanism for storing data

– Logically ordered

– Structured

– Accessible

– Direct path


Clustered Index


Clustered Index Qualities

Static Narrow

UniqueEver-

increasing


Static

Non-static key columns can reshuffle the rows in the clustered index.


Static Narrow

UniqueEver-

increasing


Narrow


Key columns from clustered indexes are included in all non-clustered indexes.

Size

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: Adds a 4-byte integer to rows non-unique clustered indexes to create unique rows.

Size

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

Ever-increasing values allows new values to be added to the end of the index structure without reorganizing rows.

100

200

300


100

200

300

600

Ever-increasing



100

200

300

400

600

Ever-increasing



100

200

300

400

500

600

Ever-increasing



CLUSTERED INDEX

Demo


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

INDEX PATTERNS


IndexIndex

PatternsSummary


Heap Patterns


• Staging tables

• Low use data

• Temporary tables

• Heavy scan data

Clustered Index Patterns

Identity Column

Surrogate Key

Foreign Key

Multi-Column


Identity Column

• Non-related value that defines each row uniquely

• Assigned when data is inserted

• Typically uses:

– Int or bigint

– IDENTITY or SEQUENCE

– Uniqueidentifier


Identity Column

Static

Narrow

Unique

Ever-increasing


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


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


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


INDEX PATTERNS

Demo


SUMMARY


IndexIndex

PatternsSummary


These are not unbreakable rules!

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

OverviewStatistics

MaintenanceTools

Analysis

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introduction to clustered indexes and heaps

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