Tracking read/unread objects in ez

For my forum efforts I figured I should offer tracking of read/unread status of certain objects. Now obviously the KISS approach would be to have a table with and just regard everything in there as being read. However, this has something like m*n storage needs and likely wont scale very far. What's would for example happen on site with 10000 objects if a new user decided all old postings should be marked read for him? It would result in the DB growing by 10000 rows...

There must be a better a way (after all there nearly always is). The only really practicable way I can think of is a table with , i.e. whole ranges of read objects. But this also suffers of two problems: 1) there's no way to keep track of different versions of an object, so it's essentially only useful for site that have archive like functionality with no changes to objects once they are published. 2) it needs a lot more code to work (especially some code that goes and merges ranges if applicable). It's not so bad but the first issue can't really be solved this way I believe...