Question

When you have values which only depends on one or more other fields +/- constants (say retail price & discount price), is it better to store those values too or to calculate them “on the fly” when retrieving the data?

Answer 1

I would agree with Tomislav – try to avoid redundancy because you can end up with data on multiple tables disagreeing with each other. It makes updates more painful.

There are exceptions that are worth considering, though, that are not related to database performance.

• When it painful to calculate the value (e.g. some complex mathematical function), then it makes sense to store (you could imagine the column as the ‘last calculated value’).
• You might have inputs that change over time, e.g. fee is derived from a fee rate, but the fee rate is stored as a single value in a configuration table. You might want to record the fee because historical fees would only be calculated from the current fee rate. Alternatively, you might store the rate by time as well to circumvent this problem.
• If the derived value can be overriden by user input or some other process, then again it makes sense to store. Alternatively, you might model this with two states ‘CALCULATED’ and ‘OVERRIDDEN’, so that you only store a value in the latter state.