1. Redundant are: xf_user_field_value which contains data from xf_user_profile(row custom_fields)
Is there a reason to have the serialized data from xf_user_profile:custom_fields also in xf_user_field_value?
2. How would I best query serialized data? How to find all users who clicked on the pizza-checkbox for both "Pizza Napoli" and "Pizza Salami"?