Optimizer and index prefixes

MongoDB University Certification Exams
1

I just did the practice exam and this was one of the answers:

All answer choices are correct!

Consider just the first choice. Any document that could match "manufacturer is Matteo AND name is Barbara AND date is 2018-07-02 would have to match date is 2018-07-02 AND, name is Barbara, and manufacturer is Matteo .

Because of this fact, the optimizer is able to rearrange the the search terms, using the existing index for each query.

To learn more, visit the following MongoDB Documentation page: https://docs.mongodb.com/manual/core/index-compound/#prefixes

This makes total sense to me, but I don’t really see it laid out in the prefix docs and I feel in the course it was explicitly outlined that the index prefix had to match in a specific order.

So based on the above indexes of

 
    "manufacturer" : 1,
    "name" : 1,
    "date" : -1
}

Can the optimizer only reorder a full compound index query like:
db.toys.find( { manufacturer : "Matteo", name : "Barbara", date : "2018-07-02" } ) or can it reorder a partially used compound index, say I just searched for:
db.toys.find( { name : "Barbara", manufacturer : "Matteo" } )

2

I’m guessing you’re referring to the M201 MongoDB Performance course?

These are the key guidelines with regards Compound Indexes:

  1. The order in which you define the index keys during index creation matter
  2. The order of the query predicates don’t matter because the query planner will rearrange the predicates in order to match it to an index that best fits
  3. The order of the sort fields matter
  4. Rules 1 to 3 apply to Index Prefixes (i.e. “partially used compound index”)

The documentation and the courses lack information around point 2… only the examples in this sub-topic somewhat hint on point 2. I got to know about this query optimisation strategy on compound indexes through practice, analysing query plans and questions in the practice exams (I think there’s one other similar question).