Hey! Nice talking to you as well.

OK. That clarifies it a bit. If you really want to measure selectivity, you add all the columns together. The leading edge, the first column, is the most important in any index, because that's what the histogram is built on, but the overall selectivity of the index is an accumulation of all columns.

This means, if you were looking at it from a query point of view, you'd be concerned about columns in the JOIN and in the WHERE clause. Generally that's not the approach taken though.