Anyway, the last episode was about SCD 4 attributes, so today it's time for...
Type 5k: Add Mini-Dimension and a Type 1 Outrigger
Last time you may remember that Kimball and Wikipedia disagreed as to what SCD 4 meant (not just in details, but completely different concepts). Well, Type 5 doesn't even exist on the Wikipedia definition, so this time we just have to deal with the Kimball definition, so that's good.
What's not so good is that Kimball's Type 5 is even worse than the type 4 - it's an optimization on an optimization and the way they do it offends one of the most basic DW design sensibilities beaten in to me by Kimball themselves - don't snowflake. We're getting ahead of ourselves though - what is Type 5? Well, in Type 4 we pull out rapidly-changing Type 2 attributes into a separate table called a mini-dimension in order to spare us some space in the main dimension. Type 5 adds an "outrigger" to this, which is basically a snowflake from the original dimension containing Type 1 versions of the attributes in the mini-dimension. Why Kimball suggest you snowflake this I can't conceive - if it's that you want it attached to the original dimension, why not just add the Type 1 attributes there? If you want it as a separate dimension, just make it separate!
Anyway, as we'll see when we get to Type 6, there are better ways of doing this any way, even if you really must have the Type 4 mini-dimension, so I'd rate this SCD type as basically a non-pattern - why waste our consciousness remembering this when we could just attach a comment to the design and learn it on-the-spot if it's ever used?
I know this is supposed to be "The Good" of the SCDs, but I find myself hard-pressed to write anything good about this design.
The series continues with a discussion of the significantly more useful (despite not introducing any new concepts), Type 6
The series continues with a discussion of the significantly more useful (despite not introducing any new concepts), Type 6
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