[eluser]jedd[/eluser]
Okay .. got it now. It'd work if the user was fairly sensible, and was going to be reading lots of threads anyway, but if the user is not very sensible and/or doesn't track the majority of the threads, then the majority of the information kept about that user's message-read status across all threads will be (mostly) pointless.
I suppose a lot does come down to the nature of users. Dang it.
The forum will be something I'd want to keep forever. I don't like the idea of nuggets of wisdom - wisgets? nugdoms? - being kept in forums, as they're notoriously hard to track down. It's especially frustrating for casual visitors who notice a Wiki link right next to the Forums link, and find the the former empty and the latter full of noise. (Most recent example for me - the MSI Wind (.net) site.) I'd like a technical approach that at least assists in the engineering solution to the social problem of getting people to 'manage their own data', as it were. Forcing deletion of threads .. is an interesting approach to encouraging users to cut-n-paste out any large, useful, insightful rambles they may have done in-thread and move them to a wiki .. but probably not very effective. At the moment I'm wanting to lock threads primarily because it's just irritating to have old threads resurrected, but very happy about the side benefit of making my message-read table leaner and better maintained.
Take your point about locked data being big compared to the message-read table, but absent a locking mechanism, the latter would always be a function of the former, which suggests something closer to a geometric than an arithmetic progression. While a lock mechanism certainly won't keep the size static - it'll reduce the rate of change of the growth.
