针对$in操作：
index early
index often
index fully
queries make use of indexes from left to right.
index sorts
If your queries will contain a sort or orderby clause, add the sorted field to your index.
commands
The order of fields in an index should be:
- First, fields on which you will query for exact values.
- Second, fields on which you will sort.
- Finally, fields on which you will query for a range of values.
先对sort field部分，进行index，这样能及时保证sort之后的顺序能够得到保证