Billions of rows btw is not a problem even on modest hardware, with a half-decent db, etc. It's analytics on this where it become possibly an issue. Retrieving one row out of 1 billion is not that much more complex than retrieving one row out of 1 million, nor is it that much more expensive computationally assuming the right index is in place.

The problem comes with high concurrency, in particular very high write concurrency, or with very complex queries which require a lot of RAM to do properly. But that's where you need a solid db, good hardware, and a solid DBA.