Re: On what OLAP can and what OLAP can't

David Cressey wrote:

> "Cimode" <cimode_at_hotmail.com> wrote in message
> news:1157639738.578212.133710_at_b28g2000cwb.googlegroups.com...
>

>>Greetings,
>>
>>Last week, while auditing a 1 year-old data warehouse totally dead in
>>terms of response time and aggregating accuracy, I had the following
>>chat with the *data multidimensional architect*...I thought some of you
>>may find it interesting to understand one sign of our time....
>>
>>Multidimensional Architect:  How come you wrote in your report that the
>>system has excessive reponse time and no accuracy because relational
>>rules of design were broken.  That show you don't understand anything
>>about OLAP modeling
>>Me: I do not need to know what OLAP modeling is.  All I can see, is no
>>matter how well designed is your OLAP system, it still incorrect result
>>at high cost because its raw data source is not normalized
>>Multidimensional Architect: But OLAP modeling has nothing to do
>>normalization, star schema's are totally denormalized...
>>Me:  Really?  I could argue that a star schema is a fully normalized
>>because it is nothing else than a fact table exclusively made up of
>>foreign key attributes pointing to dimension tables...Besides to
>>denormalize, it would initially start from some normal form?  What
>>normal form did you start from to start your *denormalizing*
>>Multidimensional Architect: That's not the point.  The point is a
>>relational model only applies to OLTP (Online Transactional Processing)
>>systems while OLAP model is targetted at Decision Support
>>Systems...OLAP is totally independent from relational model.
>>Me: What exactly is OLAP model?
>>Multidimensional Architect:  It is a multidimensional model that pulls
>>data from various relational data sources and filters only what is
>>relevant to decision makers...
>>Me:  So basically, you are stating that OLAP pulls data from SQL DBMS's
>>right?  What if the results of data sources are wrong?  What does it
>>say about the results of OLAP..Are you saying you are filtering
>>something that is not correct at the first place?  Weird...
>>Multidimensional Architect:  But OLAP does not care about the accuracy
>>of the data pulled from the RM as it gets filtered anyway.
>>Me:  How exactly do you do that?
>>Multidimensional Architect: By putting filters on the views extracting
>>data from the relational structure.
>>Me: So basically it is the same thing as putting filters on  a view to
>>attemps compensate for the loss of integrity?
>>Multidimensional Architect: No it is not the same thing?
>>Me: What is the difference then?
>>Multidimensional Architect: OLAP modeling is made of hierarchies of
>>dimensions that can be reorganized at run time...
>>Me: How many filters do you exacty put for each extracting process?
>>Multidimensional Architect: Between 6 and 10 AND conditions in the
>>WHERE clause
>>Me: And how many processes are there to filter
>>Multidimensional Architect: Between 60 and 100
>>Me: Are'nt you afraid that may have some impact on ressources consumed?
>>What if these filters are not sufficient anyway?  What if you forget to
>>add on excluding conditions in one of the numerous processes?
>>Multidimensional Architect:  That won't happen as long as I am
>>around...
>>Me: And how long will you be around
>>Multidimensional Architect: Not very long....
>>
>>The rest of the discussion was counless examples of a general trend of
>>ignorance about RM concepts by people who think in buzz words such as
>>*multidimensional modeling* while in fact it presents all
>>characteritics of good old hierarchical systems...Pathetic...

That's inevitable when a crank and an ignorant converse.

   Looking
> back, I can see myself in years gone by being on either side of the
> discussion, at different points in my career.

And your point would be?

> I can, if you like, shed some light on what the data architect might have
> been trying to show you.

He was using imprecise terms to say laughable nonsense. I am sure there are plenty of ignorants out there eager to interpret the oracle for us, but in the end, it's all still gibberish.

   However, I can only surmise that the architect's
> opinions and knowledge were somewhat like my own. They may have been quite
> different. What you might get out of such a discussion in here depends on
> what you are seeking.

In other words, we could get equal value from some fantasy conversation we each imagine.

> Since the DA isn't here, I can't rephrase what you were trying to
> communicate to the DA for that person. However, it's just vaguely possible
> that the DA had been through the same learning curve you have, and had
> grown to appreciate multidimensional modeling in addition to relational
> model, rather than instead of it.

In Cimode's case, who can really say? I see no point in trying to rationalize the irrational. If you are trying to suggest that the DA has some profound insight that most of the regular denizens of this newsgroup lack, then you are simply wrong.

> If you are seeking to genuinely understand how multidimensional modeling and
> star schema design might be genuinely useful additions to your own set of
> tools and methods, the discussion could go one way. If, on the other hand,
> you are merely looking for validation of your own opinions, and want
> justification for disregarding anything novel (to you) that the data
> architect might have said, then the discussion is likely to go nowhere, as
> many prior discussions in c.d.t. have.

Multidimensional modeling and star schemas have one advantage: the industry hopped on that buzzword-laden bandwagon a few years ago so a lot of applications and tools expect star schemas and lack facilities to handle anything that doesn't fit that exact mold.

> Before I invest the time and effort at trying to summarize and explain the
> benefits of star schema design, I want to have some inkling about whether
> it's a waste of time.

Unless you can point to some brilliant theory that makes star schemas particularly useful, it's a waste of time.

> I will comment on a few things above:
>
> First, the term "data architect" makes a great deal of sense to me. The
> terms "data multidimensional architect" or "multidimensional architect"
> sound very odd to me. I would expect any good data architect to have an
> understanding of relational data modeling, data normalization, ER modeling,
> multidimensional modeling, relational database design, star schema design,
> and (why not) object modeling, in their collection of useful tools and
> methods, and to be able to apply any one of them to good advantage, when
> appropriate.
>
> While multidimensional modeling is quite different from relational data
> modeling, it really isn't a case of "either/or" but a case of "both/and".
> Many of the religious debates in this forum are quite sterile in terms of
> comparing different tools and methods and finding different benefits and
> drawbacks to each.

When exactly did this newsgroup become comp.databases.tools.comparison ? Snake oil is snake oil. The self-aggrandizing ignorants will never trade in their various religions for mathematics and theory.

> Second, I think the data architect (DA) chose an unfortunate term when he
> described a star schema as "denormalized". A better choice of words would
> have been "not necessarily normalized (beyond 1NF)". When doing
> multidimensional modeling and star schema design, it is not so much a case
> of normalizing and then denormalizing as it is of following some design
> principles that lead to a less than fully normalized design.
>
> The multidimensional model has been dismissed as mere bunk by some acclaimed
> professionals. But I can tell you from repeated direct experience that it
> has proven very useful to me in a class of situations. And certainly
> Kimball is quite a respectable authority in this field, in his own right.

He is?!? Since when? Next you are going to try to tell us Won Kim is worthy of respect.

> Inmon, on the other hand, bases his discussion of data warehousing
> completely on normalized data as a starting place. I've never had a chance
> to apply Inmon's ideas, so I can't compare them to Kimball's ideas.
>
> I will say, however that dismissing either one of them as ignorant and
> unprofessional only shows how little the person doing the dismissing
> actually knows.

If you insist. Received on Fri Sep 08 2006 - 00:28:16 CEST