For OpenCities Map to recognize a table it must have a primary key defined. Two types of primary keys can be used: numeric or GUID (Globally Unique Identifier).

GUID is guaranteed unique. For any given feature instance in any given feature, the ID is unique for the entire table. XFM uses GUID internally to identify feature instances, so using the XFM generated ID to assign a database row ID can be useful. This avoids calling a sequence to get the value from the database or using another kind of mechanism to generate a unique numeric ID.

Non-numeric IDs have limitations and some Oracle functionality may not work properly using this type of identifier. Use the Oracle documentation to verify if any of the modules you are planning to use have issues with non-numeric primary keys.

A GUID primary key can be added to an existing table.