My first Publication Agile-Data-Warehouse-Design-eBook | Page 191

Dimensional Design Patterns for People and Organizations, Products and Services
For commercially supplied geodemographic information there may be additional
administrative or legal reasons for snowflaking. It may be supplied on a periodic
basis and updated independently of customer data, or there may be licensing
restrictions on the number of users who can access it, therefore it cannot be held in
a customer dimension available to the entire BI user community.
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Attributes that
are administered
differently may
need to be
snowflaked
Figure 6-3
Useful customer
“snowflaking”
The outriggers in Figure 6-3 do not track history. This is not a problem for first
purchase attributes as they are fixed values that can only be corrected. For
geodemographic attributes, history could be tracked by defining CUSTOMER as
HV but this would lead to uncontrolled growth in the dimension. Alternatively, the
GEODEMOGRAPHICS outrigger could be used as a mini-dimension by adding
GEOCODE KEY to existing fact tables or a newly created customer demographics
fact table.
Consequences
Outriggers complicate dimensional models and are generally unnecessary for most
dimensions. Once you have introduced useful outriggers to one dimension, your
colleagues, especially those with a 3NF bias, may be tempted to define less useful
outriggers in other dimensions that might not have such a positive effect.
You should only model outriggers that have far fewer records than the monster
dimensions they are associated with. If any attributes of a proposed outrigger
have a cardinality that approaches that of the dimension, leave them there.