Authentic MongoDB Certification Exam C100DBA Questions Answers

For a compound index to satisfy a multi-field sort, the indexed fields must occur in the same sequence as the sort specification, and their directions must either match the requested pattern or be the complete reverse of it. The query sorts by { department: 1, salary: -1 }, so option B exactly matches both the key order and the requested directions. MongoDB can scan that index and return the results already ordered, avoiding a blocking in-memory sort. Option A uses { department: 1, salary: 1 }, which does not match the mixed-direction sort and is not its full inverse. Option D puts name before the two sort keys. Because the query has no equality predicate on name, that leading key prevents the later fields from directly providing the requested global sort. Option C is not a sensible index definition for this workload. Separately, the displayed projection mixes inclusion and exclusion (name:1, salary:1, department:0), which MongoDB normally disallows except for _id; that syntax issue should be corrected independently. It does not change the intended index answer for the sort, which is B.

Study Guide reference/topic:Indexing and Query Optimization - compound indexes, sort support, index prefixes, and key direction.

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