The evolution of the data warehouse data model

[Robert Hillard]

When Ralph Kimball wrote "The Data Warehouse Toolkit" (published 1996) it defined Dimensional Modelling in a way that immediately demanded attention by data warehouse practitioners worldwide. The book and the techniques it described were not new and were common the approach we had used for the better part of a decade, what the book did do that was foundational was to describe the approach in a consistent and considered with a terminology that could be used by everyone.

There are many similar challenges that data warehouse designers face on every project. For instance two challenges we are often called upon to decide how to handle changes to source system models and the proper handling of changes to reference and master data.

The former is usually handled by splitting logical entities when creating physical tables separating attributes and relationships that have a higher probability of changing. The latter is commonly handled in one of three ways. Method one sees non volatile and volatile attributes are split into two tables (with a one to many relationship) Method two has the current attribute values are held in one table with changes over time maintained in a second table (again one to many). Finally, method three has changes across a number of concepts tracked in an audit table which is only intended for forensic purposes.

On recent data warehouse projects, we are using a variant of method one that has been formalised as “The Data Vault”.  The Data Vault techniques put forward by Dan Linstedt formalises both of these issues and makes sensible design recommendations. In particular, it adopts an approach using "hub", "link" and "satellite" tables.

Originally, Linstedt attempted to patent these concepts, but this application was rejected and he has now adopted a free approach and is promoting his concepts through books, training and his web site: http://www.danlinstedt.com/




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