by Chris Saxon, Oracle. “Why isn’t my query using an index?” is a common question people have when tuning SQL. This talk explores the factors that influence the optimizer’s decision behind this question. It introduces the concepts of blocks and the clustering factor. It discusses how these affect the optimizer's calculations. It goes on to show how these concepts work in practice using real SQL queries. This session is intended for developers who want to learn how the optimizer works and how to make their SQL run quickly! From the FreshTech 2017 conference by TechExeter www.techexeter.uk

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Why Isn't My Query Using an Index?
An introduction to SQL performance
Chris Saxon, Oracle Developer Advocate for SQL
chris.saxon@oracle.com
@chrisrsaxon & @sqldaily
blogs.oracle.com/sql
www.youtube.com/c/TheMagicofSQL

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Why isn’t Oracle using my
#@!%
index?!

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5. Colour BAG Colour BAG Colour BAG Colour BAG Colour BAG
Blue 73 Brown 56 Green 11 Orange 19 Red 2
Blue 31 Brown 40 Green 31 Orange 19 Red 41
Blue 1 Brown 41 Green 70 Orange 85 Red 21
Blue 64 Brown 65 Green 4 Red 42 Red 32
Blue 69 Brown 23 Green 80 Red 100 Red 12
Blue 85 Brown 21 Green 28 Red 70 Red 83
Blue 84 Brown 73 Green 44 Red 38 Yellow 29
Blue 40 Brown 97 Green 99 Red 33 Yellow 31
Blue 41 Brown 57 Green 32 Red 63 Yellow 51
Blue 88 Brown 63 Orange 85 Red 22 Yellow 33
Blue 100 Brown 100 Orange 21 Red 80 Yellow 4
Blue 6 Brown 80 Orange 76 Red 80 Yellow 9
Blue 64 Brown 75 Orange 32 Red 40 Yellow 27
Blue 90 Green 20 Orange 93 Red 68 Yellow 48
Blue 24 Green 50 Orange 70 Red 54 Yellow 56
Brown 88 Green 85 Orange 2 Red 93 Yellow 15
Brown 50 Green 17 Orange 15 Red 93 Yellow 81
Brown 69 Green 27 Orange 5 Red 20 Yellow 83

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What’s this got to do with
?

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M&M => a row
Document => an index
A bag => a block

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Method #1 => TABLE ACCESS FULL
Method #2 => INDEX RANGE SCAN
Multiblock reads
Single block reads

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Source: Pixabay
Which is faster?

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FULL TABLE SCAN
Visit each bag once
INDEX RANGE SCAN
Visit each bag ~10 times

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What if there’s a
Promotion?
silver sweets in 1% of bags

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FULL TABLE SCAN
Visit each bag once
INDEX RANGE SCAN
One bag visit

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The
Cost Based Optimizer
estimates these calculations

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DEMO

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"M & Ms (3685102458)" by Greg Willis from Denver, CO, usa Uploaded by russavia. CC BY-SA 2.0

16. Colour BAG Colour BAG Colour BAG Colour BAG Colour BAG
Blue 1 Brown 7 Green 13 Orange 19 Red 25
Blue 1 Brown 7 Green 13 Orange 19 Red 25
Blue 1 Brown 7 Green 13 Orange 19 Red 25
Blue 2 Brown 8 Green 14 Red 20 Red 26
Blue 2 Brown 8 Green 14 Red 20 Red 26
Blue 2 Brown 8 Green 14 Red 20 Red 26
Blue 3 Brown 9 Green 15 Red 21 Yellow 27
Blue 3 Brown 9 Green 15 Red 21 Yellow 27
Blue 3 Brown 9 Green 15 Red 21 Yellow 27
Blue 4 Brown 10 Orange 16 Red 22 Yellow 28
Blue 4 Brown 10 Orange 16 Red 22 Yellow 28
Blue 4 Brown 10 Orange 16 Red 22 Yellow 28
Blue 5 Brown 10 Orange 17 Red 23 Yellow 29
Blue 5 Green 11 Orange 17 Red 23 Yellow 29
Blue 5 Green 11 Orange 17 Red 23 Yellow 29
Brown 6 Green 11 Orange 18 Red 24 Yellow 30
Brown 6 Green 12 Orange 18 Red 24 Yellow 30
Brown 6 Green 12 Orange 18 Red 24 Yellow 30

17. Colour BAG Colour BAG Colour BAG Colour BAG Colour BAG
Blue 1 Brown 3 Green 4 Orange 7 Red 9
Blue 1 Brown 4 Green 4 Orange 8 Red 9
Blue 2 Brown 5 Green 5 Orange 9 Red 9
Blue 2 Brown 5 Green 6 Red 1 Red 9
Blue 3 Brown 5 Green 6 Red 2 Red 10
Blue 4 Brown 6 Green 7 Red 2 Red 10
Blue 6 Brown 7 Green 8 Red 3 Yellow 1
Blue 7 Brown 8 Green 8 Red 3 Yellow 1
Blue 7 Brown 8 Green 10 Red 4 Yellow 2
Blue 8 Brown 9 Orange 1 Red 4 Yellow 2
Blue 9 Brown 9 Orange 2 Red 5 Yellow 3
Blue 9 Brown 10 Orange 3 Red 6 Yellow 5
Blue 10 Brown 10 Orange 3 Red 6 Yellow 5
Blue 10 Green 1 Orange 4 Red 7 Yellow 6
Blue 10 Green 2 Orange 4 Red 7 Yellow 7
Brown 2 Green 3 Orange 5 Red 8 Yellow 8
Brown 2 Green 3 Orange 6 Red 8 Yellow 9
Brown 2 Green 3 Orange 6 Red 8 Yellow 10

18. Clustering factor

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clustering factor = 10 (good)

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clustering factor = 120 (bad)

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DEMO

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How do I
Improve
the clustering factor?

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Tannin Heap Simon A. Eugster CC-BY-3.0
By default
tables are

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Organized
Part ition ed
Clusters
How do I ensure
consistent
clustering?

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There can be only 1!

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Colour
Weight

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Cost
Rows
Full Table Scan
Index Range Scan 1
Index Range Scan 2
Index Range Scan 3

28. Rows
“Toy Blocks (6831034024)” welcometolearn CC BY 2.0

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