Non-persistent attributes in persistent entities

Currently you can only choose persistency on entity level. It would be great if (for a persistent entity) you could specify on attribute level whether this attribute needs to be persisted to the database or not.  

Motivation:
Sometimes you want to introduce attributes solely for display purposes, for example an enumeration to switch between certain views of the same entity or for attributes that are calculated before opening a page (which have not been configured as calculated attributes because of performance reasons). Currently you either have to add these attributes in a persitent entity, which results in unneccessary columns in your database table, or create a non-persistent helper entity (which means additional work).