This will be very inefficient due to the way DBMS commonly lay out data in pages. And if you want to do any kind of aggregate queries (e.g. analytics) you're pr

This will be very inefficient due to the way DBMS commonly lay out data in pages. And if you want to do any kind of aggregate queries (e.g. analytics) you're probably in for some royal pain. If you want to do this for security, why not layer the DB behind some system that requires and verifies the users access tokens for each request? The only situation where such a setup might make sense is when you actually need per-user migrations to cater to specific customer's needs, but then you'll make it very hard to work with all customer's data through a generic interface.