The popular advice in these comments to use indexes sounds really good. I haven't thought it out fully, but my first thought was "this sounds like a DIY columna

The popular advice in these comments to use indexes sounds really good. I haven't thought it out fully, but my first thought was "this sounds like a DIY columnar store". A columnar database does a lot of this work for you. I'm not 100% it's a good option in this case, because things like email subjects are not good columnar data. I would like to know more about how problematic that is.