While these functions work with phpbb they don't really pertain to the database issue I'm describing.
Essentially, every time someone posts, I would take a few keywords from their topic title and get a list of 5 or so related topics using the current forum's topic list. So I would either have to query the database every single time someone clicks new topic, which I can get around by making a json (or a lot of jsons per forum) for the forum. Then I'd only need that json updated a few times a day by the server itself.
But going that route I'm not too sure how to avoid needing a massive json, or multiple jsons. I was hoping there was a better way.