How many tables is too many? Maintaining unique user-added metadata without risk of 'cross-contamination'

I have an online repository that has catalogs of document collections that anyone with an account can access. The documents themselves are image files as well as electronic file that have been 'streamed to PDF'. All documents have 'objective' metadata associated with them such as date_submitted or doc_type or some date_mentioned or it's reference_number or page_count.  Users though are clamoring for the ability to add their own *subjective* metadata and I think this sounds like a great idea to enhance the usability and attractiveness of the site. My problem in getting started is where to store and how to store this user-specific metadata. It's scope regulation is just as important and tied into the general users/group administration functionality for collaboration purposes on the one hand and privacy concerns on the other.  At first I thought, hey, just have a table for each user - maybe all kept in a separate database, altogether distinct from the one containg the 40 or so tables governing/relating to the repository database itself. The more I think about it though the more I think that this approach sound inelegant or inefficient although I don't have any adata to this effect - nor have I conducted any simulations or stress testing experiments. What do the experts think? Maybe keep the user-specific metadata in XML files on disk or as image/blob fields in a table?Start Free Trial