Reader question — Facts, dimensions, and picking a grain

Reader question on Facts, dimensions, and picking a grain, and the answer belongs somewhere more findable than an email.

Dimensional modelling is thirty years old and still the right default, because it optimises for the thing that actually matters — an analyst being able to answer a question without asking anyone.

The question was, roughly, "when does this stop applying?" — which is the right question and the one lessons routinely fail to answer. Every technique has a range of validity, and stating it is what separates a lesson from a recipe.

It sits in the Modelling the warehouse module, and the exercise runs against retail-orders. That pairing is deliberate: the dataset was built with the trap the lesson describes already in it, so the exercise fails in the instructive way rather than the confusing one.

Ordering matters here more than in most courses. It follows Schema drift, and the contract that prevents it and leads into Slowly changing dimensions, and the "as of when" problem, and reading it out of sequence mostly works but costs you the setup.

Free means free, and it also means we can rewrite it whenever it is wrong. No edition, no errata PDF, nothing to repurchase. The whole course is 36 lessons and the fixes land the day we find them.