Should I store a calculate value in my database along with the variables?

Question

In all the applications I have made where a database is used I typically store the calculated value along with the variables needed to calculate that value. For example, if

Answer 1

This violates Normal Form to have this calculated value. Unless there is a reason to denormalize (usually performance constraints) then you should make every attempt to normalize your tables, it will make your database much easier to maintain/improve and denormalize may lock you into a design that is difficult to alter easily and exposes your data to inconsistencies and redundancy.

Answer 2

The standard approach is not to store this kind of calculated values - it breaks normalization.

There are cases you want to store calculated values, if it takes too long to recalculate, or you are running a data warehouse etc. In your case, you want stick to the normalization rules.

Answer 3

In my experience, the most common thing to do is to a) store the calculated value, b) without any CHECK constraints in the database that would guarantee that the value is correct.

The right thing to do is either

• don't store the result of the calculation
• store the calculated value in a column that's validated with a CHECK constraint.

MySQL doesn't support CHECK constraints. So your options are

• don't store the result of the calculation
• switch to a dbms that supports CHECK constraints, such as PostgreSQL.

Answer 4

It all depends on what resources are scarce in your environment. If you do pre-calculate the value, you'll save CPU time at the cost of increased network usage and DB storage space. These days, CPU time is generally much more abundant than network bandwidth and DB storage, so I'm going to guess that as long as the calculation isn't too complicated then pre-calculating the value is not worth it.

On the other hand, perhaps the value you're calculating takes a substantial amount of CPU. In this case, you may want to cache that value in the DB.

So, it depends on what you have and what you lack.

Answer 5

Simple math is relatively cheap, however you need to weigh up the additional storage cost vs performance saving when storing these values. Another thing you may want to consider is the affect this will have on data updates, where you cant simply just update the field value, you need to update the calculated value too.