Understanding MongoDB Aggregation

Question

Pretend that our collection has 1M document
Now if i do aggregation like this:

db.users.aggregate({
 $group: {
 _id : "$gender",
 avgAge : { $avg : "$age" }
 }
})

then how it will prepare the data for me?
Say i have a index on the id but not on the gender so is it gonna read all the 1M documents? if so then how does it scale?

Answer 1

Indices can speed up an aggregation when utilized in a $match or a $sort stage at the beginning of a pipeline.

With your example, no index will be used, since you don't use either. So every document will be accessed.