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Message Posted: Wed, 14 Nov 2002 @ 00:46:47 GMT


     
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Subj:   Re: 3NF vs Dimensional modelling
 
From:   Neil Raden

Duncan,

You wrote:

  A dimensional model CAN be in a RDBMS, OR a multidimensional DBMS (e.g. express etc).  


I don't agree this with. WHen we talk about Dimensional Models, we're talking about relational databases. A MULTIdimensional model, if you can find it, lives in a multidimensional database like Express, Essbase, Powerplay, etc. I say "if you can find it" because they aren't really models, they are mixed up with the data and you cannot abstract the model from the instance.

I'd also make the distinction for a very important reason - the DM is relational and, hence, inherits all the power of a relational model. The models inside these MOLAP tools are much more limited, almost laughable. That's why we call them cubes, it isn't meant as a compliment. They have no attributes, they only have independent dimensions. In fact, about all the information you can encode into a dimension is the name of the member and who its parent is, no attributes. Many allow only one roll-up per dimension, can't handle ragged or split hierarchies, multi-parent children or cross-dimensional attributes, roles or qualities. All of these can be handled easily in DM.


  When a relational model is talked about we can pretty much take it as read that a 3NF EDW is being referred to (and looking at the context it is clear that is the case) - harping on about this trivial point is worthless.  


I think I've just explained why it isn't trivial, but that's most likely because most RDB practitioners don't really understand the dimensional model.


  An OLTP schema is one that is built to satisfy short sharp tactical queries. A data warehouse is not built for that purpose and is therefore categorically NOT an OLTP schema. If you don't like using 3NF come up with something less misleading - I suggest "3NF EDW" - can't get that wrong!  


Then please characterize for me, if a data warehouse schema is NOT OLTP, but IS 3NF, but is NOT dimensional, then just what is it? What differentiates it from the two polar opposities? I've never heard this disclaimer before, that a 3NF EDW schema is not an OLTP schema. Can you be specific? (And I'm not being sarcastic, I'd really like to know)


  And also, using a dimensional model as your EDW is fine if your enterprise can be fully modelled by this rather simple method (i.e. your enterprise is not complex).  


You clearly don't know the DM, or you could not make this outrageous claim. I've been modeling RDB's for 20 years, and using the dimensional model for 12 (there aren't many people still alive who can make that claim), and I can assure, it is NOT simple and I've modeled some very complex organizations with it. But rather than make that claim, perhaps you can prove your point with a situation that is too complex for the DM.


  If representing the relationships within your business IS complex I suggest you will encounter the shortcomings of using a sole dimensional model very quickly.  


What is a "sole" DM???? Would you make the same claim about a "sole" 3NF EDW model?


  So some businesses will encounter less pain than others when they only have a DM with no underpinning EDW. Fully modelling retail product sales is much easier than fully modelling telco sales -  


Experience would suggest otherwise. I've modeled telcos with DM. Some of those models are 7 or 8 years old, holding up very nicely.


  the businesses are just very different in complexity (and for that matter data volumes - another driver for the creation of dependent data marts).  


OK Duncan, this is where you've really put your foot in your mouth. You just said that high data volumes, such as a telco, lead to dependent data marts. I presume you mean that that is a good thing. But why is it? Because, the 3NF EDW data warehouse is TOO BIG TO USE (excuse me Teradata people, I realize that this isn't as much of a constraint as it is in the other databases). So what you do is build this big database, because you think that's what you ought to do, but then you create this huge mess of downstream data marts, different schema which means more ETL and more maintenance, for users to query.

There are DM warehouses out there in the 10's of terabytes. Don't believe the claims that it can't be done.


-NR

Neil Raden
Hired Brains, Inc.
www.hiredbrains.com



     
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