Filter by a second element of composite key in Couchbase

Question

I have two fields a & b, I have created a view with emit([doc.a, doc.b])

Now I want to search only by field b, how can I do that.

I tried startKey = ["", 48], endKey = [{}, 49], but this gives incorrect results,, it just totally ignores the second element of the keys and instead just filters on the first element.

I am trying to know if I can serve filtering on [a], [b], [a and b] from a single view, as I require large number of filters and their combinations. So can a single view handle filter combinations?