We've already seen how Couchbase handles primary indexes for documents. The key in the key/value pair is the unique primary key for a document. Using the key/value API, documents may be retrieved only by this key. While this might seem limiting, there are key/value schema design patterns that help to provide flexibility. We'll explore them in Chapter 5, Introducing N1QL.

Fortunately, Couchbase as a document store provides a much more powerful approach for finding your documents. To illustrate the problem and the solution, we'll walk through a brief example. Imagine having a simple JSON document such as this:

{
  "Username": "jsmith",
  "Email": "[email protected]"
}

The key/value limitation is easy to see. Imagine we want to find a user by their username. The key/value solution might be to use the username as the key. While that would certainly work, what happens when we also want to query a user by their e-mail address? We can't have both e-mail and username as a key...