Kevin, The Hadoop ecosystem is a lot more complex than just a simple key-value store, but a key-value store is sufficient to answer your question. Let's say you have data of the form "Key => Value1", in one location, and "Key => Value2" in another location. If you know one value, it's not trivial to find all related values. Unless.... you have an inverted index that allows you to look up the key for any given value, and then use that key to look up other values. For instance, say I have a database that lists the mailing address for each person. e.g. Kevin => 1 Apple St, Sean => 2 Zebra Ln This is great if you just want to see where specific people live, but what if your question starts with having an address and needing to know all the people who live there? Instead of the key being the name and the value being the address, you create a different index that inverts this: e.g 2 Zebra Ln => Sean, 1 Apple St => Kevin Now it's easy to see everyone who shares an address because they would also share a key (which is actually not doable in some key-value stores - in which case you would modify the value field to encode a sequence of values). I know it's been a while since you asked your question, but I hope this helps!
... View more
