tedster - 10:02 pm on Mar 22, 2013 (gmt 0)
Unanswered question: Is there one underlying index or two?
Here's a Google patent from 2007 that may be relevant to the question. It's inscrutable in many ways (more so than even other patents) but it seems to me to hint at an answer.
System and method for selectively searching partitions of a database [patft.uspto.gov]