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The evolution of the data warehouse data model
by 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 consistent with 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 way 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. One method one sees non volatile and volatile attributes are split into two tables (with a one to many relationship).  Another method has the current attribute values are held in one table with changes over time maintained in a second table (again one to many). The final common approach has changes across a number of concepts tracked in an audit table which is only intended for forensic purposes.

In my book, Information-Driven Business, I spend considerable time explaining the underlying approaches to modelling structured data and to measuring the benefits of different approaches.

Over recent data warehouse projects, we are using a variant of the first method of handling history 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, there was an attempt to patent these concepts, but this application was rejected and he has now been able to adopt a free approach and is promoting his concepts through books, training and his web site: www.danlinstedt.com.

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