In MongoDB, is there a performance difference between $in covering all possible values vs leaving the field out in a compound index?

Working with Data
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I’m working with a MongoDB compound index like this:

db.users.createIndex({ gender: 1, city: 1, age: 1 })

Suppose city has only a few possible values, e.g., “Chicago”, “LA”, “NY”.

I can query either by leaving city out:

db.users.find({ gender: "M", age: { $gt: 25 } })

or by explicitly specifying all possible cities in $in:

db.users.find({ gender: "M", city: { $in: ["Chicago","LA","NY"] }, age: { $gt: 25 } })

From what I understand:

  • Leaving city out makes MongoDB internally use [MinKey, MaxKey] for the index scan.

  • Using $in with all possible values essentially covers the same range.

My questions:

  1. If $in includes all possible values for a field, is there any performance difference compared to leaving the field out?

  2. Does the number of possible values matter in this case, or will MongoDB treat both queries the same in terms of index scanning?

  3. Are there any cases (e.g., very large datasets, range queries) where one approach might be preferable?

I want to understand if manually specifying all values in $in is ever beneficial when the field is fully covered.

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