[Boost-users] [multi_index] composite_key question

I've a multi_index container that has a composite_key index which is used to express a "checker board" like concept. I need to iterator over all rows, from 0 to n. I know how to employ equal_range(make_tuple(y)) to get the elements within a specific row, but how do I determine n? The only way I can think of doing this is doing a linear search using another index. Is there a better way?