Course note — Why your query costs what it costs

Revised Why your query costs what it costs this week. The change was small and the reason was not.

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 first draft explained the mechanism and stopped. What was missing was the failure: the thing that happens when you get it wrong, described concretely enough that someone recognises it later. A lesson that only teaches the correct version leaves you unable to spot the incorrect one, which is the situation you will actually be in.

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.