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Message Posted: Tue, 12 Nov 2002 @ 17:52:06 GMT


     
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Subj:   Re: 3NF vs Dimensional modelling
 
From:   Ken Hansen

Hi Anon,

So far the responses to your question have asserted 3NF to be better than Dimensional models(Dm) for the core DW but let me highlight two key reasons, both relating to foreseeable but unpredictable evolution:

- the business management will undoubtedly change their requirements as to detail and summaries; changes to Dm usually mean dissembling and reassembling cubes with additional data from source archives (probably themselves in 3NF)

- the business enterprise will also change size and shape - acquiring and/or selling divisions or simply restructuring vertically/horizontally/geographically, again data can be added/filtered out far more easily when in a relational database.

Additionally, something most advocates of Dm do not mention is that all but the most simple enterprises will need several fact tables with their own dimensions to cater for different sections of the business - hence a lot of the fact table columns get replicated with implications for space and maintenance. That is why Dm is recommended for DataMarts and not DW.

Dm's also become problematic when data is updated more frequently because calculation of the summary rows usually requires hours to rebuild the data cube(s) instead of simply adding new rows to a relational table. I can think of several where recalculation of summaries takes most of the night and creeps into the working morning.

My personal experience is that the value of the DW to Business Analysts grows exponentially when the analysts can have extra data added to compare/filter core data with that from external sources or internal scenarios - try that with Dm if you have the time and skills.

Finally, do not underestimate the skills of business analysts. If you do the they will simply use the DW as a source for their own databases built on office servers - SQL_Server, My_SQL and MS_Access have lots of advocates and DW managers should enlist them as internal champions of the DW rather than have them work and deride the DW as loose cannons!


God Luck.

Ken Hansen



     
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