Reader question — Why your query costs what it costs

Reader question on Why your query costs what it costs, and the answer belongs somewhere more findable than an email.

Columnar storage, partitioning, and file layout explain most of the difference between a query that returns in two seconds and the same query taking four minutes and forty dollars.

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. The exercise is a thinking one rather than a query one — some of this material is about deciding what to compute, and that does not need a keyboard.

Ordering matters here more than in most courses. It follows Slowly changing dimensions, and the "as of when" problem and leads into One metric, one definition, 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.