I feel this somewhat generalizes. "Cases where a simple list/array is better than a hashed collection." Which, oddly has the perverse problem of "when you sho

I feel this somewhat generalizes. "Cases where a simple list/array is better than a hashed collection." Which, oddly has the perverse problem of "when you should give up on a single collection of your data, and embrace storing it in different ways." That is, few folks seem to internalize or consider that every index is essentially a copy of the data. A complete copy, if you are doing a fully projected index. So, when is it better to just do a full scan over something else? Well, its probably never better in isolation of every other concern. But, when is it better to just do a scan of the data over duplicating it in a place that would let you hash in? Depends on how much extra work you are doing because of it? Having typed that, really I think the trick is to ask yourself roughly (not exactly, amusingly) what it means to add an index. Or to use a hashtable. Or to use a list and a hashtable. And realize that the question of using an index is the same as the question of keeping another copy of your data somewhere.