Originally Posted by penagate
Yeah, usually. They tend not to be reliable in the least.
The method you suggest (a table with a unique index on thread ID + IP address) would be more reliable, but also fallible. What about dynamic IPs? What about proxies? The table might get fairly large, but that may or may not be a problem.
Another method would be to store thread ID, user ID, and view count, which a unique index on thread ID and user ID and counting guests as user 0. This would allow you to track who viewed the thread and how many times they did so.
MySQL SUM() may or may not be cached (I forget) so you could also store a total number of views against the thread record if performance is a concern; display that, then provide an option to drill down to views against users.
(Sorry, I wrote this when I read your reply, then went to lunch and forgot about it.)