Indexes: When Column Selectivity Is Not Always A Requirement

While preparing for my SQL Saturday Salt Lake City presentation, Climbing the B-Tree, I ran into one of the “word of mouth” facts that the first column of an index should be highly selective, i.e., it should only point to one or a few rows. This is not always the case and I’ll show you why below.

In the past, I’ve said many, many times that when you define an index that the first column should be highly selective. I’ve continued with statements like Gender is not a good data field because it is not very selective. Either you have