ponder this for a moment if you will.
This is how i plan on producing the same effect on my new forum, without using a new table.
on your threads table, add a text field on the end of your table.
When a new thread is started, add the users ID into that field. When someone reads that new thread, their id is added onto the end of the text field deliminated by a comma. When you come to reading whether the thread has been read or not by said user, gather the content of the text field, explode it by commas and perform an if is in array check to deturmin whether the members id is in the there. If it is, post is read, if not, post is unread.
When a new reply is made, replace all the ids with only the new posters id back in there.
surely that has to be quicker than querying another table x amount of times looking to see whether a thread has been read.
Just food for thought
