When to Denormalize a Database Design

Question

I know that normalization has been extensively discussed on Stack Overflow. I\'ve read many of the previous discussions. I\'ve got some additional questions though.

I

Answer 1

Your senior developer makes extremely valid points. I've learned these the hard way myself by servicing systems that don't de-normalize the historical data.

In a sense it's not really adding any overhead to the database. You are creating invoice tables from existing data in the database. An invoice is a snapshot in time. De-normalizing the information you need to produce that invoice can make your reporting SO much easier. When you are required to produce a new report and expected to do it quickly you will appreciate the de-normalization.

In terms of having total in the database. This has saved my ass before when I've made a change to an application that caused numbers not to add up the same way (not as hard as you may think). On a live application the totals gave me a definite place to return to in order correct the discrepancies. I've written about this before, you can read it here: http://jlrand.com/?p=95