There is no way to assign a default name to primary Key for newly created entities
A macro needs to be installed which will easily enable renaming of primary keys in your existing entities.
1. Download and install the macro from the following link: http://modeling.inside.quest.com/entry.jspa?externalID=4631&categoryID=34
2. Select entities in the Physical Model Explorer, on Workspace or in the result set of the Object Viewer.
3. Right click any of the selected entities and choose Macros | Rename Primary Keys. See output in Message Explorer/Log. If you do not select any entity, click the Macros menu and choose Productivity | Rename Primary Keys.
4. The macro will change PK names in all entities (after your confirmation).
