Query scaling costs

Provides power to do joins on the global database.
The web usually can't afford order-n queries - too many web pages.
Google gives a sense of this scale:

• centralized DB, but still useful measure.
• order-n at index time (input).
• faster at query time.

Can be expensive if not smart, careful or lucky.

Classic relational database optimizations are tricky with remote, distributed data sources.