Download Where Are My Keys

Where Are My (Primary) Keys?
Ami Levin
Mentor,
[email protected]
Think Big. Move Fast.
We Always Did it This Way…
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Where are my (primary) keys?
Session Goals
Revisit one of the fundamental design
principals of relational databases - key
selection.
Explore the controversies associated with it
from a very practical, hands-on perspective,
with a special emphasis on some surprising
performance issues that may arise from suboptimal selection of keys...
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Where are my (primary) keys?
What is a Primary Key?
The Forefathers
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Where are my (primary) keys?
Normalization
“A relation whose domains are all simple can be
represented in storage by a two-dimensional column
homogeneous array of the kind discussed above.
Some more complicated data structure is necessary
for a relation with one or more non-simple domains.
For this reason (and others to be cited below) the
possibility of eliminating non-simple domains appears
worth investigating. There is, in fact, a very simple
elimination procedure, which we shall call
normalization”
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Where are my (primary) keys?
1st Normal Form
 There's no top-to-bottom or left-to-right
ordering to the rows and columns
 There are no duplicate rows
 Every row-column intersection contains
exactly one value
 There are no repeating groups
 All columns are regular
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Where are my (primary) keys?
1st Normal Form
SSN
Name
Phone Number
123-45-6789
Muammar Gaddafi
+218-00-9876
987-65-4321
Bashar Assad
+1-202-6543221
SSN
First Name
Last Name
Phone Number
123-45-6789
Muammar
Gaddafi
+218-00-9876,
+218-00-8765
987-65-4321
Bashar
Assad
+1-202-6543221
SSN
First Name
Last Name
Phone Number 1 Phone Number 2
123-45-6789
Muammar
Gaddafi
+218-00-9876
987-65-4321
Bashar
Assad
+1-202-6543221
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Where are my (primary) keys?
+218-00-8765
2nd Normal Form
 R is in 1NF.
 Given any candidate key K and any
attribute A that is not a constituent of a
candidate key, A depends upon the whole
of K rather than just a part of it.
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Where are my (primary) keys?
2nd Normal Form
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Order ID
Line Number
Customer
0001
1
FISSA
0001
2
FISSA
0002
1
PARIS
OrderDetailID
OrderID
LineNumber
Customer
AC934245FF00B
0001
1
FISSA
8BA50CC2044AF
0001
2
FISSA
F00B344923AB4
0002
1
PARIS
Where are my (primary) keys?
3rd Normal Form
 R is in second normal form (2NF)
 Every non-prime attribute of R is non-transitively
dependent on every candidate key of R.
A non-prime attribute of R is an attribute that does not
belong to any candidate key of R.
A transitive dependency is a functional dependency in
which X → Z (X determines Z) indirectly, by virtue of X →
Y and Y → Z.
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Where are my (primary) keys?
3rd Normal Form
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Order ID
Line Number
Product
Manufacturer
0001
1
Chair
IKEA
0001
2
Gum
Mentos
0002
1
Fighter Jet
Boeing
Where are my (primary) keys?
Keys







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Simple
Composite
Candidate
Primary
Artificial / Surrogate
Intelligent
Natural
Where are my (primary) keys?
The Debate
To ID or not to ID?
 IDENTITY (1,1) vs. Natural key
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Where are my (primary) keys?
Pro Artificial (I)
In some cases, no natural key exists
and an artificial key is the only option.
 Examples?
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Where are my (primary) keys?
Pro Artificial (II)
Natural keys can change.
Artificial keys never change.
 How Often?
 Cascading referential constraints
 Artificial keys can change
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Where are my (primary) keys?
Pro Artificial (III)
Natural keys may be long and complex.
 Become longer with each level
 900 Bytes limit in SQL Server
 Multi-column joins
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Where are my (primary) keys?
Pro Artificial (IX)
Artificial keys help improve
performance.
 Simpler join predicates
 Ever increasing clustering effect
 Short keys = Smaller DB = Faster
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Where are my (primary) keys?
Pro Artificial (X)
Artificial keys reduce clustered index
fragmentation.
 Minimize maintenance down time
 What about deletes?
 What about non-clustered indexes?
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Where are my (primary) keys?
Pro Natural (I)
Natural keys have business meaning.
 Artificial keys are never queried for
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Where are my (primary) keys?
Pro Natural (II)
Queries on tables using natural keys
require fewer joins.
 The more familiar and meaningful the key,
the less joins are required
 “Bypass” joins
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Where are my (primary) keys?
Pro Natural (III)
Data consistency is maintained
explicitly when using natural keys.
 Artificial keys enable logical duplicates
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Where are my (primary) keys?
Pro Natural (IX)
Natural keys eliminate potential
physical clustering performance issues.
 Contention for clustered regions
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Where are my (primary) keys?
Less Mentioned Issues (I)
Artificial keys are the de-facto standard.
 ORMs generate artificial keys
 LINQ doesn’t cache composite key rows
…
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Where are my (primary) keys?
Less Mentioned Issues (II)
Data statistics and optimizations.
 Statistics on artificial keys are useless for
parameter sniffing
 Estimations on composite key statistics
are less accurate
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Where are my (primary) keys?
Less Mentioned Issues (III)
Modularity and portability.
 Migration to other platforms
 Merging with other databases
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Where are my (primary) keys?
Less Mentioned Issues (IX)
Simplicity and aesthetics.
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Where are my (primary) keys?
Demo Spec
A database of web sites.
 URL
 Country and city of owner
 Country ISO code for external app
 Data consistency is crucial
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Where are my (primary) keys?
Natural vs. Artificial Keys
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Where are my (primary) keys?
Ask Yourself
 Is there a natural key that I can use as a primary
key?
 Are there a few natural candidates?
 Which one is the simplest and most familiar?
 How stable is it?
 How will it be used logically?
 What will be the physical access patterns for this
table?
 What are the common query types for this table?
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Where are my (primary) keys?
For More Information
 A Relational Model of Data for Large Shared Data Banks (E.F.
CODD)
 The Relational Model for Database Management: Version 2 (E.F.
Codd)
 An introduction to database systems (C.J. Date)
 Database in Depth: Relational Theory for Practitioners (C.J.
Date)
 The Database Relational Model: A Retrospective Review and
Analysis (C.J. Date)
 Joe Celko's Data and Databases: Concepts in Practice (J. Celko)
 Joe Celko's SQL for Smarties, Fourth Edition: Advanced SQL
Programming (J. Celko)
 Database Modeling and Design, Fifth Edition: Logical Design
(T.J. Teorey, S.S. Lightstone, T. Nadeau, and H.V. Jagadish)
 Pro SQL Server 2008 Relational Database Design and
Implementation (L. Davidson, K. Kline, S. Klein, and K. Windisch)
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Where are my (primary) keys?
Where Are My Keys?
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Where are my (primary) keys?
Where Are My Keys?
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Where are my (primary) keys?