Detailed Analysis or Simple Analysis?
It is vital that you get this right, because the decision on what constitutes the fact data for the data warehouse will depend on these answers. This is one of the most important decisions you will make in the whole project. If you get the level of detail wrong for the fact data you may ultimately have to scrap the whole data warehouse and start again. Just to reinforce this statement, and to ensure that it is not taken as a throwaway comment, if you get the level of detail wrong for the fact data you will probably have to scrap the whole data warehouse and start again. The whole design, the sizing, the capacity planning and so on will be based on this decision. If the level of detail is incorrect, the hardware will be the wrong size, the database design and layout will be incorrect, and any partitioning you are using will be wrong. You will need to start over.
If you are aware that, as in the example above, the level of detail will increase later, you can design with that in mind, and ensure that any design you put in place now should not hamper that change later. Having an understanding of these details will allow you to round out the query requirements. It allows the user requirements to be rationalized, and a reasonable estimate of the sorts of queries that are possible in the future to be calculated.
