Free Practice Questions for MongoDB C100DBA Exam

The proposed index is a compound index across home_score and away_score. MongoDB supports compound multikey indexes, but it imposes an important restriction: for any one indexed document, no more than one field covered by the compound index may contain an array. Document 1 is valid because only away_score is an array. Document 2 is also valid because only home_score is an array. Document 4 contains scalar values for both fields. Document 3 contains arrays in both indexed fields: home_score: [5,8] and away_score: [9,6] . Indexing that document would require a compound multikey index to expand two independent array dimensions, which MongoDB prohibits. Therefore document 3 prevents the compound index from being valid. The restriction is important because a single multikey field already creates multiple index keys per document; allowing multiple independently array-valued keys in the same compound index would create ambiguous and potentially combinatorial key combinations. DBAs designing indexes over document schemas with arrays must understand these multikey constraints before building compound indexes. The failing document is _id:3, making option D correct.

Study Guide reference/topic:Indexing and Query Optimization - multikey indexes, compound multikey restrictions, and array-valued fields.

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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.

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Encryption at rest protects persisted database data on storage media, but it is not a complete defense against every form of data exposure or breach. MongoDB ' s encrypted storage capabilities protect files and storage-level data, yet applications and the database server must still process authorized data in usable form. A compromised application account, stolen database credentials, excessive privileges, or a malicious authorized user can therefore expose data even when the underlying storage is encrypted. Network traffic also requires separate transport protection such as TLS; encryption at rest does not secure data as it travels between clients and servers. Option B incorrectly describes transport encryption rather than an at-rest capability. Option A is too subjective and not an inherent limitation, particularly in managed deployments where storage encryption may be enabled automatically. Option D is similarly overstated; encryption adds cryptographic work, but severe performance impact is not an intrinsic defining limitation. The correct security model is defense in depth: combine at-rest encryption with TLS, strong authentication, least-privilege authorization, auditing, and secure key management. Among the choices, C best captures the limitation that at-rest encryption alone cannot prevent all data breaches.

Study Guide reference/topic:Security, Networking, and Encryption - encryption at rest, TLS, authorization, and defense in depth.

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