Comment by Sohcahtoa82

Such a database could be sharded out insanely easily, too. Rather than having 1 BEEFY server with a couple TB of RAM, you could do a couple dozen servers with a more modest 256 GB, with each server having a strictly defined subset of serial numbers (ie, one server could handle notes with serial numbers ending in 00 or 01, another handling 02/03, etc.), and the load balancing becomes extremely simple.