Introduction to Clustered Indexes and Heaps

transcript

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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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

THE HEAP

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 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.

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.

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.

Ever-increasing

CLUSTERED INDEX

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

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

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Introduction to Clustered Indexes and Heaps

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