Indexing and Performance

What is the primary benefit of using a Covered Query in MongoDB?

Refer to the exhibit. What does this output indicate about the query performance?

A DBA notices that a query on the products collection uses an index scan but returns only a small fraction of documents. The index is { category: 1, price: 1 }. The query filters on category and price with a range condition on price. Which statement best describes why the index might still be efficient despite scanning many index entries?

Refer to the exhibit. Which index is most effective for this query?

Which situation is best suited for a Multikey Index?

What is the consequence of having a high number of 'keysExamined' compared to 'nReturned' in a query?

When evaluating index performance, what is the impact of a 'partial index'?

A collection 'events' has a compound index { tenantId: 1, createdAt: -1 }. A developer runs db.events.find({ tenantId: "acme", createdAt: { $gte: ISODate("2024-01-01") } }).sort({ createdAt: -1 }).limit(50). The explain output shows the index is used but the query still performs a large in-memory sort. Which action resolves the sort performance issue?

A DBA is troubleshooting a slow query on a large collection. Which two techniques directly reduce the number of documents the query must examine for an equality-plus-range predicate? (Choose two.)

An e-commerce application frequently runs a query filtering by category and sorting by price descending. Which index structure provides the optimal performance for this access pattern?

Refer to the exhibit. An application queries active user documents frequently, but explainPlan output reveals a COLLSCAN stage. What is the most appropriate remediation step?

A DBA observes that a query with filter { a: 5, b: { $gt: 10 } } and sort { c: 1 } is using a collection scan despite the existence of an index { a: 1, b: 1, c: 1 }. The DBA wants to improve performance. Which action is most likely to allow the index to support both the filter and the sort?

When would you choose to create a Partial Index instead of a standard index?

Which TWO of the following scenarios are best handled by a Hashed Index?

A DBA needs a query that returns only the fields name and email from a 'users' collection, filtered by an equality on email, to avoid fetching full documents from disk. Which index design supports this efficiently?

An operations engineer notices that a frequently executed aggregation pipeline fails with a 'Exceeded memory limit for $sort' error. Which index configuration best resolves this memory constraint?

A DBA is designing indexes for a collection that stores user activity logs. The collection has fields: user_id, action, timestamp, and metadata (an embedded document). The most common queries are: (1) find all actions for a given user_id sorted by timestamp descending; (2) find all users who performed a specific action within a time range. Which TWO indexes would best support these queries? (Choose two.)

When evaluating the performance of a 'hint' in MongoDB, what should you keep in mind?

A collection 'logs' receives continuous inserts and is queried by both timestamp ranges and a rarely used severity field. A DBA creates six indexes to cover every query variant, and now insert throughput has dropped sharply while index sizes dominate the working set. Which action best restores insert throughput while preserving the important query paths?

A support team reports that a MongoDB 6.0 replica set is experiencing slow queries on a collection where almost every document has a unique value for the field status. A developer suggests creating a hashed index on status to speed up equality lookups. Which outcome should the DBA expect?

Refer to the exhibit. The explain output shows a SORT stage wrapping an IXSCAN stage. What is the performance implication of this execution plan?

An application frequently queries a large collection using two fields: 'category' (equality) and 'timestamp' (range). Which index strategy provides the most efficient execution plan?

A sharded cluster experiences poor query performance because a targeted query targeting a specific shard key still results in a scatter-gather operation across all shards. What is the root cause?

Why should you avoid creating too many indexes on a single collection?

A DBA is reviewing a query that performs a sort on a field named score in descending order. The collection has an index { score: 1 }. The query also includes a filter on a field named active with an equality condition. The DBA observes that the query planner chooses a collection scan and performs an in-memory sort. Which index would allow the query to use an index for both the filter and the sort?

Which property of a field makes it a poor candidate for an index?

Refer to the exhibit. A developer attempts to run a query using both 'tags' and 'location' in a single $and operation. What is the expected behavior regarding index usage?

A DBA needs to optimize a query that filters on a field with high cardinality, such as email, and also sorts on a low cardinality field, such as country. The query is { email: "user@example.com" } with sort { country: 1 }. Which index would best support this query?

An application frequently queries a large collection by 'user_id' and 'status', sorting the results by 'timestamp'. Which index provides the most efficient execution plan?

A MongoDB DBA observes that a query with filter { status: "active", created_at: { $gte: ISODate("2024-01-01") } } and sort { created_at: -1 } is performing a collection scan. The collection has an index { status: 1, created_at: -1 }. Which of the following best explains why the index is not being used efficiently?