This seems fine to me as proposed, assuming 255 bytes is enough. I don't see much point to separating the prefix from the rest of the URL here.
Topic on Talk:Multi-Content Revisions/Database Schema
255 bytes should be enough, but if it costs nothing, let's just declare 1023 bytes or something. But this does make a difference for indexes, right? Do we need an index on this?
If the varchar is more than 255 bytes, it uses an extra byte per row.
It also makes the rows potentially larger, although I don't know to what extent that might be a concern.
It may also affect indexing, if you decide to put an index on it. I can't think of any reason why an index would be necessary though except for potential maintenance.
ok, 255 bytes it is for now.