The tables in the operational part of the Boundary database (meaning
non-warehouse tables) suffix primary key columns and foreign key columns
with `_id`. Many of these columns appear in the warehouse dimension and
fact tables as normal attributes, not primary keys or foreign keys. To
disambiguate ID columns from operational tables, the primary key column
for dimension tables will be renamed from `id` to `key` and the foreign
key columns in the fact tables will be renamed to change the suffix from
`_id` to `_key`.
