Quoting Ethan Fremen <ethan@acquia.com>:
a.) it makes it impossible to shard the DB because you have to coordinate what the next sequence ID is, a huge problem with scaling. b.) it makes it difficult to merge two (or more) existing DBs, whether development/production or otherwise. c.) it means that every drupal site has namespace collisions all over the place with every other drupal site.
I think you go about the problem incorrectly. Any merging of data would need to use the Drupal API to do that merge. You therefore do not need to know what the next id is because you simply shouldn't care. When merging the data from DB-2 into DB-1 you do not supply the id's to the data from DB-2 and allow Drupal to create new id's in DB-1. As for your supposed namespace collisions between sites, that is the nature of all data when two merge to one. It is the job of the one doing the bridging to overcome that issue but overcoming that issue shouldn't involve needing to make your id's 64bit. What do you do with users who have subscribed to both sites? What do you do with nodes with matching titles? The dilemma you describe can only be resolved by the rules set forth by the business acquiring DB-2. Earnie -- http://for-my-kids.com/ -- http://give-me-an-offer.com/