Let’s say you’re designing the DB schema for the next stack overflow and more specifically the part of the schema that handles question ratings.
I assume you’d use a table like:
ratings(question_id, user_id, rating)
… that will both record ratings and make sure no user votes twice on the same question.
That table alone could handle rating data but it might result in slow queries.
Taking performance into consideration, would you consider storing the sum of ratings for each question in the questions table, even though this data would be redundant since it’s derivative from the data in the ratings table?
I would generally first start with a normalized model, not de-normalizing the sum of ratings in the question table.
Then, when the application is working well enough, I would do some performance testings, to determine whether the application handles load good enough — compared to the load I expect to have in production.
If it doesn’t handle load well enough, I would check for bottlenecks — and correct the most important ones, until the application does well.
Once the application is in production, if the website has lots opf users, it’ll be time to make some additionnal optimizations.
To make things simple :
In the end, yes, maybe, de-normalizing the sum of ratings to the questions table might help ; but do you need to do it ?
That is the real question 😉