I love how your use of the word redundant demonstrated redundancy.

I still think a table and extra processing is wasteful and uneccesary. Rather, instances should just do what they do with communities and users.

The host instance decides the number. Remote instances use that number followed by @instance. This already fits in with the federated model, as posts are always on the community's local instance originally, and comments are always on the user's local instance originally.

If you're on lemmy.one and reply to a post in a community in lemmy.world, that post should be lemmy.world's number @lemmy.world, then your comment should be lemmy.one's number @lemmy.one, rather than each instance picking its own sequential number for both.

This kind of thing would probably be the easiest change to implement while requiring minimal server overhead.

For a specific example, to me, this post is https://lemm.ee/post/1726780. In the original, federated instance, the post is https://lemmy.ml/post/2308622. Instead, it should appear to me as https://lemm.ee/post/2308622@lemmy.ml.

Your comment, to me, is https://lemm.ee/comment/1409174. The federated link is https://lemmy.one/comment/1393053. This is the link you see, and should continue to see, but I should see https://lemm.ee/comment/1393053@lemmy.one.

Using the instance in the numbering scheme means each federated host instance manages its own numbering while ensuring there will never be a clash between instances. https://lemm.ee/comment/1393053@lemmy.one and https://lemm.ee/comment/1393053 would refer to two completely different comments. This information is already being transmitted in the process of federation, so it's just a matter of tagging the data when it's received.