Exam C100DBA Topic 4 Question 46 Discussion

When MongoDB ' s winning query plan uses a B-tree index, the plan includes an IXSCAN stage. That stage contains the index-specific information a DBA needs to identify how the query is being serviced, including the key pattern, index name, traversal direction, and index bounds. For example, keyPattern can show a compound definition such as { status: 1, createdAt: -1 }, making it straightforward to confirm which indexed keys participate in the chosen plan. A FETCH stage indicates that MongoDB is retrieving full documents, often after an index has produced candidate record identifiers; the fetch stage itself does not identify the index key. A COLLSCAN stage means MongoDB is scanning the collection and therefore is not using a normal index scan for the predicate. GROUP is unrelated to index selection. Effective DBA query analysis usually involves inspecting both queryPlanner.winningPlan and execution statistics such as totalKeysExamined, totalDocsExamined, and nReturned. But for the narrow question of which stage exposes the index key used by the winning plan, IXSCAN is the correct answer. Therefore option C is correct.
Study Guide reference/topic:Indexing and Query Optimization - explain output, winning plans, IXSCAN, key patterns, and index bounds.