Microsoft SQL Server Relational Databases and Primary Keys

Where Are My (Primary) Keys?
Ami Levin
Mentor,
ALevin@SolidQ.com
Think Big. Move Fast.
Presented to the
San Francisco SQL Server User Group
September 2013

GEOData Database
The GEOData database that is used for the
demo can be downloaded here:
http://sdrv.ms/Z8ZmNb
Ami Levin – SolidQ2 |

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 sub-
optimal selection of keys...

What is a Primary Key?
Ami Levin – SolidQ

The Forefathers

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”

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

1st Normal Form
SSN Name PhoneNumber
123-45-6789 MuammarGaddafi +218-00-9876
987-65-4321 Bashar Assad +1-202-6543221
SSN First Name Last Name PhoneNumber
123-45-6789 Muammar Gaddafi
+218-00-9876,
+218-00-8765
987-65-4321 Bashar Assad +1-202-6543221
SSN First Name Last Name PhoneNumber1 PhoneNumber2
123-45-6789 Muammar Gaddafi +218-00-9876 +218-00-8765
987-65-4321 Bashar Assad +1-202-6543221

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.

2nd Normal Form
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

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.

3rd Normal Form
Order ID Line Number Product Manufacturer
0001 1 Chair IKEA
0001 2 Gum Mentos
0002 1 Fighter Jet Boeing

Keys
 Simple
 Composite
 Candidate
 Primary
 Artificial / Surrogate
 Intelligent
 Natural

The Debate
To ID or not to ID?
 IDENTITY (1,1) vs. Natural key

Pro Artificial (I)
In some cases, no natural key exists
and an artificial key is the only option.
 Examples?

Pro Artificial (II)
Natural keys can change.
Artificial keys never change.
 How Often?
 Cascading referential constraints
 Artificial keys can change

Pro Artificial (III)
Natural keys may be long and complex.
 Become longer with each level
 900 Bytes limit in SQL Server
 Multi-column joins

Pro Artificial (IX)
Artificial keys help improve
performance.
 Simpler join predicates
 Ever increasing clustering effect
 Short keys = Smaller DB = Faster

Pro Artificial (X)
Artificial keys reduce clustered index
fragmentation.
 Minimize maintenance down time
 What about deletes?
 What about non-clustered indexes?

Pro Natural (I)
Natural keys have business meaning.
 Artificial keys are never queried for

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

Pro Natural (III)
Data consistency is maintained
explicitly when using natural keys.
 Artificial keys enable logical duplicates

Pro Natural (IX)
Natural keys eliminate potential
physical clustering performance issues.
 Contention for clustered regions

Less Mentioned Issues (I)
Artificial keys are the de-facto standard.
 ORMs generate artificial keys
 LINQ doesn’t cache composite key rows
 …

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

Less Mentioned Issues (III)
Modularity and portability.
 Migration to other platforms
 Merging with other databases

Less Mentioned Issues (IX)
Simplicity and aesthetics.

Demo Spec
A database of web sites.
 URL
 Country and city of owner
 Country ISO code for external app
 Data consistency is crucial

Natural vs. Artificial 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?

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)

WhereAre My Keys?

Microsoft SQL Server Relational Databases and Primary Keys

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