When are computed columns appropriate?

Question

I\'m considering designing a table with a computed column in Microsoft SQL Server 2008. It would be a simple calculation like (ISNULL(colA,(0)) + ISNULL(colB,(0))) - like a tot

Answer 1

Make Sure You Are Querying Only Columns You Need

I have found using computed columns to be very useful, even if not persisted, especially in an MVVM model where you are only getting the columns you need for that specific view. So long as you are not putting logic that is less performant in the computed-column-code you should be fine. The bottom line is for those computed (not persisted columns) are going to have to be looked for anyways if you are using that data.

When it Comes to Performance

For performance you narrow your query to the rows and the computed columns. If you were putting an index on the computed column (if that is allowed Checked and it is not allowed) I would be cautious because the execution engine might decide to use that index and hurt performance by computing those columns. Most of the time you are just getting a name or description from a join table so I think this is fine.

Don't Brute Force It

The only time it wouldn't make sense to use a lot of computed columns is if you are using a single view-model class that captures all the data in all columns including those computed. In this case, your performance is going to degrade based on the number of computed columns and number of rows in your database that you are selecting from.

Computed Columns for ORM Works Great.

An object relational mapper such as EntityFramework allow you to query a subset of the columns in your query. This works especially well using LINQ to EntityFramework. By using the computed columns you don't have to clutter your ORM class with mapped views for each of the model types.

var data = from e in db.Employees
           select new NarrowEmployeeView { Id, Name };

Only the Id and Name are queried.

var data = from e in db.Employees
           select new WiderEmployeeView { Id, Name, DepartmentName };

Assuming the DepartmentName is a computed column you then get your computed executed for the latter query.

Peformance Profiler

If you use a peformance profiler and filter against sql queries you can see that in fact the computed columns are ignored when not in the select statement.