Understanding SQL Server Indexes - 2013-11-19

Understanding Indices
Richard Douglas

“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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Indexing strategies
It’s all about DWI knowledge:

D W I
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Dell Software

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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Pre Requisites
Understanding the SQL Server Data Page
• A data page is 8KB (8192 bytes)

Page header
1 - Cooper
2 - Hofstadter
3 - Koothrappali
5 - Wolowitz
4 - Kripke

• Page header is 96 bytes leaving
8096 bytes for data

• Slot array stores the logical order

543 21
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Index
Architectures

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Dell Software Group

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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Golden Rules for Clustered Indexes
Bad example; Surname, Firstname, Middle Initial
• Narrow
• Static
• Progressive

Impacts nonclustered indexes
Fragmentation

• Unique
• Fixed width

Space impact

• Not Null

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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The importance of maintenance
Page split scenario
Page pointers
Page header
Data Row 1
Data Row 2
Data Row 3
Data Row 5
Data Row 4


Page header

Data Row 4
Data Row 5

5 321

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Page header

Data Row 6
Data Row 7
Data Row 8
Data Row 9
Data Row 10
54

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Fragmentation
Two main types
• 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
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Solution Area

Product

Description

Backup and
Recovery

Fast, flexible backup and
recovery with industry-leading
compression technology

Performance
& Operations

Discover and resolve
performance issues in production
before they impact end users and
service levels
Deepest possible understanding
of database performance and
norms

Performance
Tuning

Plan and develop applications
that deliver both functionality and

Development

optimal performance
Comprehensive schema, object,
security and change
management

Administration

Community,
Knowledge,
Training

Project
Lucy
© 2012 Quest Software Inc. All rights reserved.

Community crowdsourcing for
SQL Server tracing and
performance information!
Pg. 27

SQLRelay Dates
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@SQLRelay2013

11/11/13 – Reading
12/11/13 – Southampton
13/11/13 – Cardiff
14/11/13 – Birmingham
15/11/13 – Hemel Hempstead
25/11/13 – Newcastle
26/11/13 – Manchester
27/11/13 – Norwich
28/11/13 – Bristol
29/11/13 – London

Any questions?

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Thank you
Richard.Douglas@Software.Dell.com
@SQLRich

Understanding SQL Server Indexes - 2013-11-19

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Understanding SQL Server Indexes - 2013-11-19