What are design patterns to support custom fields in an application?

Question

We develop a commercial application. Our customers are asking for custom fields support. For instance, they want to add a field to the Customer form.

What are the kn

Answer 1

if those 'extra' fields are incidental and don't care to do searches on them, I usually go for option 2 (but like JSON better than XML). If there's going to be searches on custom fields, option 3 isn't hard to do, and usually the SQL optimizer can get reasonable performance out of it.