"I've been very critical of multitenancy at the application layer," Ellison said during his keynote speech on Tuesday. He noted that security tools would not work sufficiently in this architecture.

"Data isolation should be at the infrastructure layer -- in separate virtual machines -- or at the database layer," he said. Of the two options, the database layer is more efficient, Ellison argued.

The pluggable database is inherently more efficient for a number of reasons, Rajamani later explained. Today, a server running 100 databases must switch rapidly between all those databases, which creates a lot of overhead. "A hundred binaries are going to 100 context switches," he said. A single database eliminates all this task switching. Also, each database has a large number of background processes that need to run continuously, and reducing all of these processes to a single set saves resources.

Also, having a single copy of database software to handle multiple user databases reduces the amount of disk space needed, since it eliminates the multiple copies of the host database software. (Oracle did not disclose how this new architecture would affect licensing costs.)