In compound index, are missing in-between fields always translated to [MinKey, MaxKey] and is index usage guaranteed if the leading field is present?

Working with Data
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I’m experimenting with compound indexes in MongoDB and noticed some behavior I want to clarify.

Suppose I have a compound index like this:

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

When I run a query that specifies the first and last field but skips the middle one, for example:

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

The indexBounds in the explain plan shows something like:

gender: [ "M", "M" ]
city: [ MinKey, MaxKey ]
age: (25, MaxKey]

This seems to indicate that MongoDB automatically substitutes missing in-between fields with [MinKey, MaxKey] so the index can still be used.

My questions are:

  1. Is it guaranteed that whenever a middle field in a compound index is missing from the query, MongoDB will substitute [MinKey, MaxKey] in the indexBounds?

  2. As long as the leading field of a compound index is included in the query, is it guaranteed that the index can be used?

  3. Are there any exceptions to these behaviors (e.g., with range queries, sort, partial indexes, or newer MongoDB versions)?

I want to understand whether this is a fundamental rule of how MongoDB compound indexes work, or if it’s just an observed implementation detail.