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It has become fashionable to regard the final deployment step of building an extract-transform-load (ETL) system as a mere tactical choice between delivering dimensional relational tables or OLAP cubes into the end-user environment. But is this choice quite so superficial? Should we defer thinking about this choice until just before roll-out? In this article we take a hard look at this final deployment conundrum and urge you to resolve this question very early in the design process Business intelligence (BI) developers have largely accepted the premise that data is most user-friendly when delivered in a dimensional format. If you define the data warehouse as the platform for all forms of BI, then the last step of the ETL process in the data warehouse is exposing the data in a dimensional format. Many BI developers have recognized that a properly designed set of dimensional relational tables can be transformed into an OLAP cube in a virtual one-to-one mapping. For various reasons explained below, I recommend that all OLAP cubes be built directly from dimensional models. The dimension tables in such a relational schema become the OLAP cube dimensions, often referred to as the edges of the cube. The fact tables from the relational schema provide the content of the OLAP cube cells.
While there can be some small semantic differences between the relational dimensions and the OLAP cube dimensions, the profound overlap between these two approaches has made it tempting to regard the final deployment choice as a tactical maneuver executed at the very end of the data warehouse development. Worse yet, the argument is sometimes made that BI applications can be switched between relational and OLAP implementations because of this similarity.
Not so fast! Under the right circumstances, the ETL pipeline can be mostly insulated from the final deployment choice, but the relational vs. OLAP choice is multi-faceted decision with lots of issues to consider. Let’s look at the advantages and disadvantages of both choices before we jump on either bandwagon.
Equally Easy in Either Approach
Making the Final Choice
So how do senior management and the enterprise BI system designers resolve the final deployment conundrum: Dimensional relational or OLAP? As I hope you appreciate, there is no slam dunk answer since there are significant advantages and disadvantages for both approaches. But let’s consider two extremes. If you are a large distributed enterprise with a number of different database vendors and you are struggling to establish more commonality across your BI deployments, as well as creating an enterprise wide pool of BI development expertise without being beholden to any single vendor, then I recommend dimensional relational. On the other hand, if you are looking for the most potent single-subject solution with high performance and killer analytics, and you are confident that you can source the development expertise you need, then I recommend OLAP supported by underlying dimensional relational schemas. Otherwise, it depends.
Ralph Kimball, founder of the Kimball Group, teaches dimensional data warehouse and ETL design through Kimball University and reviews large warehouses. He has four best-selling data warehousing books in print, including The Data Warehouse ETL Toolkit. Write to him at email@example.com.