I missed the post join part, and going to be honest, I don’t know how to do that. I try to avoid anything that is likely to slow a site and adding extra joins in db queries can really slow down a WP site.
There isn’t an easy way to update all of the existing posts. Basically, you need to do a query, get all the posts, loop though them, get the related post and update the values. If you have a lot of posts on your site doing this will likely just timeout the browser trying to do it.
