>> but to use it throughout your db eats up tons of space
I think human resources and outages cost a lot more than disk space these days. If I can trade disk space for either of those I will do it in a blink of an eye.
Not just disk space, larger index size mean less fits in caches and memory, more I/O etc.
These are keys which are typically used constantly by an app in the db with joins etc. They are some of the hottest data typically and you want to avoid cache misses on them if possible.
I think human resources and outages cost a lot more than disk space these days. If I can trade disk space for either of those I will do it in a blink of an eye.