Nullable fields in fact tables

Why should fact table fields not be nullable?

Three big reasons:

1. Null key fields can’t have descriptions from a domain table attached (without special processing in the report/query)
   
2. Nulls can cause wrong group by/drilling results.
   
3. Using natural keys gives the optimizer more information to choose faster query paths

Kimball has thoughts on cases where fact table fields should be nullable in his latest design tip.