Question

If I have a composite key (primary or plain index) based on fields, say Year(int) and Ord(int), will filtering WHERE Year=2011 benefit from that key or will it ignore it completely?

Answer 1

If you filter on the n left-most columns (where n >= 1), then that index could at least be used (might still be ignored due to other facts).

So if your index is on (Year, Ord) and you filter by Year – then yes, that index might be used.

However, if you filter on Ord alone, that filter will not even be considered – it cannot be used.