The OP didn't
suggest how many years his data covers, but it's quite possible that
pulling a full year's worth of data will read enough of the table that
there's no point in worrying about whether an index could be used
anyway.
There are only a few years worth of data, 2008 - 2012. However, the data consists of 15 min measurements and when renormalized (un-pivoted) is several hundred million records. It is conceivable that someone will want to query by month, or even hour of the day.
