YMMV but yes, for our application they were very significant. In a marketplace app with a significant number of sellers, most queries will be highly selective i

YMMV but yes, for our application they were very significant. In a marketplace app with a significant number of sellers, most queries will be highly selective if only because they'll usually be filtered by seller. One of the top queries will be "show me my all-time sales". If your only index is time, you will touch your whole database every single time a customer asks for it...