Understanding IndicesRichard Douglas

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“Can’t you just put an index on it?”

(Name withheld to protect the guilty)

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Fixing the problem

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What about Europe?

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You need the big picture

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It’s all about DWI knowledge:Indexing strategiesD

W

I

ata

orkload

nternals

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Your host

• Richard Douglas

• Systems Consultant

• Editor in Chief - ToadWorld.com

• SQL Server MCITPro

• PASS Chapter Leader

• Blog: http://SQL.RichardDouglas.co.uk

• Twitter: @SQLRich

• Email: Richard.Douglas@Software.Dell.com

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Agenda

• Pre requisites

• Understanding indexes and their architecture

• Utilising SQL Server DMO’s

• Index golden rules

• The importance of maintenance

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Understanding the SQL Server Data PagePre Requisites

• A data page is 8KB (8192 bytes)

• Page header is 96 bytes leaving 8096 bytes for data

• Slot array stores the logical order

Page header

1 - Cooper

1

2 - Hofstadter

2

3 - Koothrappali

3

4 - Kripke

54

5 - Wolowitz

5

10 Dell Software Group

Index Architectures

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Index Architecture

Heaps

Clustered Indexes

Nonclustered Indexes

Nonclustered Columnstore Indexes

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Heap Structure

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Clustered Index Structure

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Nonclustered Index Structure

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Columnstore pages

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Index options

Fill factor

Pad Index

Filtered index (WHERE)

Include

Data Compression

Online

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Hidden dangers

Versioning

Uniqifiers

VARCHAR

NULLS

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Introducing Index DMO’s

• Admin– Sys.dm_db_index_physical_stats

• Usage– Sys.dm_db_index_usage_stats

– Sys.dm_db_index_operational_stats

• Missing– Sys.dm_db_missing_index_details

– Sys.dm_db_missing_index_columns

– Sys.dm_db_missing_index_groups

– Sys.dm_db_missing_index_group_stats

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Missing Index Limitations

• Not intended to fine tune an indexing configuration.

• Only gathers 500 missing index groups.

• Optimal column orders for the index are not specified.

• In some scenarios it has been known to only return information on included columns.

• Returns only raw information about columns.

• Does not suggest filtered indexes.

• It can return different costs for the same missing index group that appears multiple times in XML Showplans.

• Multiple similar indexes may be returned.

• Only considers non-trivial query plans.

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Bad example; Surname, Firstname, Middle InitialGolden Rules for Clustered Indexes

• Narrow

• Static

• Progressive

• Unique

• Fixed width

• Not Null

Impacts nonclustered indexes

Fragmentation

Space impact

This example will have 13 bytes of overhead alone;• 4 byte uniquifier• 2 byte variable offset + 6 bytes for variable length fields• 1 byte for NULL values and NULL bitmap

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Golden Rules for NonClustered Indexes

• Have an optimal clustered key

• Not narrow

• Reduce unnecessary overhead– Fixed Width

– Not null

• Consolidate

• Index foreign keys

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Page split scenarioThe importance of maintenance

Page header

Data Row 1Data Row 2Data Row 3

Data Row 4

321

Data Row 5

5

Page header

Data Row 4Data Row 5

54

Page header

Data Row 6Data Row 7Data Row 8

Data Row 10

109876

Data Row 9

Page pointers

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Two main typesFragmentation

• Logical / External

“When the next logical page is not the next physical page”– Significantly reduces scan

performance

– Can cause extra IO

• Physical / Internal

“When page density is sub optimal”

– Increased storage space

– More room required in the buffer pool

– More IO’s to read in the data

– More logging, bigger backups

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Fixing fragmentation

• Rebuild Indexes

• Reorganize Indexes

• Options– Maintenance plan

– Custom/community scripts

– Third party tools

• Guideline (sys.dm_db_Index_Physical_Stats provides these figures)

– 0 – 5% No action

– 5 – 30% Reorganize

– 30% + Rebuild

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Maintenance best practices

• Auto Close

• Auto Shrink

• Statistics

• Recovery models

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Summary

Architecture

Options

Querying

Design

Fragmentation

Maintenance

Pg. 27© 2012 Quest Software Inc. All rights reserved.

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Any questions?

Thank you

Richard.Douglas@Software.Dell.com@SQLRich