The optimal index for a given query can usually be found with the equality-sort-range rule, which 
indicates that in an index, the equality fields should be first, then the sort fields, and finally the range
fields.
So, for example, let's consider this query: db.test.find({a:'x', b: {$gt: 10}}).sort(}}{{{c:1}}}
Here, `a` is an equality based match, `b` is a range based match, and `c` is used for sorting. Following 
the equality-sort-range rule, the most optimal index for the above query will be: `{a:1, c:1, b:1}`