When is it better to store flags as a bitmask rather than using an associative table?

Question

I’m working on an application where users have different permissions to use different features (e.g. Read, Create, Download, Print, Approve, etc.). The list of permissions i

Answer 1

Your queries will run faster using a flags enumeration (bitmask), because you won't need to include a join to the associated table in order to make sense of the value.

Answer 2

Personally, I would use an associative table.

A bitmask field is very difficult to query and join on.

You can always map this to your C# flags enum and if performance becomes and issue refactor the database.

Readability over premature optimization ;)

Answer 3

There is no definitive answer, so do what works for you. But here is my catch:

Use option 1 if

• You expect permissions to grow to many
• If you might need to do a permission check in the database stored procedures itself
• You do not expect millions of users so that records in the table do not grow massively

Use option 2 if

• Permissions are going to be limited to handful
• You expect millions of users