Free MongoDB C100DBA Practice Exam with Questions & Answers | Set: 4

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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db.collection.stats() returns statistics for a specific collection, including storage-related values that help a DBA understand that collection ' s disk footprint. Therefore, for a collection named order, db.order.stats() is the relevant command among the choices. The result can include metrics such as logical data size, allocated storage size, document count, index information, and related collection statistics depending on version and storage engine. Option A, db.stats(), reports database-level statistics aggregated across the database rather than isolating the order collection. db.hostInfo(), option C, reports host and operating-system information rather than collection storage consumption. db.serverStatus(), option D, provides broad server-process runtime metrics such as connections, memory, operation counters, replication/storage-engine information, and other health data, but it is not the collection-specific disk-utilization interface requested here. Administrators can also use dedicated helpers such as collection storage-size or total-size methods when they need a more narrowly defined measurement. For a general collection-level statistics query targeting order, however, db.order.stats() is the correct answer, making option B correct.

Study Guide reference/topic:Performance Monitoring and Tuning - collection statistics, storage utilization, db.collection.stats(), and database/server metrics.

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