Mongodb Aggregation Framework: Does $group use index?

@ArthurTacca, as of Mongo 4.0 $sort preceding $group will speed up things significantly. See https://stackoverflow.com/a/56427875/92049.

Per Mongo's 4.2 $group documentation, there is a special optimization for $first:

Optimization to Return the First Document of Each Group

If a pipeline sorts and groups by the same field and the $group stage only uses the $first accumulator operator, consider adding an index on the grouped field which matches the sort order. In some cases, the $group stage can use the index to quickly find the first document of each group.

It makes sense, since only the first entry in an ordered index should be needed for each bin in the $group stage. Unfortunately, in my 3.6 testing, I haven't been able to get nearly the performance I would expect if the index were really being used. I've posted about that problem in detail in another question.

EDIT 2020-04-23

I confirmed with Atlas's MongoDB Support that this $first optimization was added in Mongo 4.2, hence my trouble getting it to work with 3.6. There is also a bug preventing it from working with a composite $group _id at the moment. Further details are available in the post that I linked above.