Having spent most of the last decade mostly in DB2 (both mainframe and server), and lessly in SQL Server, I've come to take schemas for granted and get confused when they're not there. The question of total addressable data is less clear. I'm not sure that any of the schema supporting databases engines can hold as much data per schema as other databases do per database. So, while schemas allow for segregating tables by some factor, they don't necessarily allow for larger integrated databases.