What is the correct index for the specified collection structure in MongoDB?

Question

MongoDB

There is a collection of objects, an approximate structure:

{
  "a": unix timestamp,
  "b" : string,
  "c": usefull data ..
}

And there is a request of the form -- give the number of objects with:

(a >= t1 && a <= t2) && (b in [b1, b2, b3, ..., bN]

In this case, N can be large (1 thousand objects).

Which index will be the most efficient?

Answer 1

this question is best answered by explain : https://docs.mongodb.com/manual/reference/operator...
depends on how many records are left to iterate after filtering on "a"
depends on how many unique rows " b"
in short, build an index on "a" and on "b" and see which one is greater/less