Why does direction of index matter in MongoDB?

Question

To quote the docs:

When creating an index, the number associated with a key specifies the direction of the index, so it should always be 1 (ascendin

Answer 1

The simple answer that you are looking for is that the direction only matters when you are sorting on two or more fields.

If you are sorting on {a : 1, b : -1}:

Index {a : 1, b : 1} will be slower than index {a : 1, b : -1}