C100DEV Indexing — All Questions With Answers

Question 1mediummultiple choice

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An application frequently queries a collection using equality conditions on 'status' and 'category', followed by a range filter on 'created_at', and sorts the final results by 'priority' descending. How should you design a compound index for optimal performance according to the ESR rule?

Question 2mediummultiple choice

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An application queries an array field named 'tags' containing strings for product categorization. Queries frequently search for documents matching multiple tags simultaneously. Which index strategy optimizes this query workload?

Question 3mediummulti select

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A developer needs to ensure that the 'email' field in a 'users' collection is unique across all documents. Which TWO statements accurately describe the behavior or requirements of unique indexes in MongoDB?

Question 4hardmultiple choice

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You have a collection with 10 million documents. Most documents have a 'status' of 'processed', but about 1% have a 'status' of 'pending'. You frequently query for 'pending' documents sorted by 'creation_date'. Which indexing strategy provides the best balance between write performance and query efficiency?

Question 5mediummultiple choice

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Refer to the exhibit. Based on the explain output provided for a query on the 'logs' collection, which of the following is true about the query performance?

Exhibit

{
  "queryPlanner": {
    "winningPlan": {
      "stage": "FETCH",
      "inputStage": {
        "stage": "IXSCAN",
        "keyPattern": { "userId": 1, "timestamp": -1 },
        "indexName": "userId_1_timestamp_-1",
        "direction": "forward"
      }
    }
  },
  "executionStats": {
    "nReturned": 50,
    "totalKeysExamined": 50,
    "totalDocsExamined": 50
  }
}

Question 6mediummulti select

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You are implementing a session management system using MongoDB. Which TWO of the following are true regarding the use of TTL (Time-To-Live) indexes?

Question 7hardmulti select

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You have an index defined as { last_name: 1, first_name: -1 }. Which TWO of the following sort patterns can be efficiently satisfied by this index?

Question 8hardmulti select

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A developer wants to use a Wildcard Index to handle highly dynamic metadata fields. Which THREE of the following are limitations or characteristics of Wildcard Indexes?

Question 9hardmulti select

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Which TWO of the following statements regarding wildcard indexes in MongoDB are correct?

Question 10mediummultiple choice

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Refer to the exhibit. Which compound index would be most efficient for this query?

Exhibit

db.users.find({age: {$gt: 25}, status: 'active'}).sort({name: 1})

Question 11mediummulti select

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Which THREE of the following are valid limitations or behaviors of hashed indexes?

Question 12mediummultiple choice

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Refer to the exhibit. Which of the following queries can be served efficiently by this index?

Exhibit

db.products.createIndex({category: 1, price: -1})

Question 13mediummultiple choice

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An application frequently queries a collection of user profiles using an equality condition on the status field followed by a range condition on the created_at timestamp field. Which compound index structure provides optimal ESR (Equality, Sort, Range) rule compliance?

Question 14mediummultiple choice

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An application frequently queries a collection of user profiles using an equality condition on the 'status' field, followed by a range condition on the 'age' field, and then sorts the resulting documents by 'joinDate' in descending order. Which single index structure follows the ESR rule most effectively?

Question 15mediummultiple choice

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An e-commerce application frequently queries a collection by filtering on category and sorting by price descending. You need to create an index that efficiently supports both the equality query and the sort operation without requiring an in-memory sort stage. How should you define the index key pattern?

Question 16hardmultiple choice

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A collection contains large documents with deeply nested arrays. You want to create an index on a field inside these arrays to support queries matching multiple elements simultaneously. Which index type provides this capability while preventing Cartesian explosions in index size?

Question 17mediummultiple choice

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A collection `inventory` has an index `{ category: 1, price: 1 }`. A developer runs `db.inventory.find({ price: { $gt: 100 } }).sort({ category: 1 }).explain('executionStats')` and notices a high number of documents examined. They want to reduce the number of examined documents while keeping the same index. Which index usage strategy should they apply?

Question 18mediummultiple choice

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A social media application has a `posts` collection with a field `tags` that is an array of strings. You create an index on `{ tags: 1 }`. A query retrieves posts that contain both "mongodb" and "database" in the `tags` array. Which statement accurately describes how MongoDB uses this index for the query?

Question 19hardmulti select

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A development team is designing an index strategy for a MongoDB collection that stores sensor readings. Each document has a `sensor_id` field, a `timestamp` field, and a `value` field. The team needs to support queries that filter on `sensor_id`, filter on a range of `timestamp`, and project only `sensor_id` and `timestamp`. They also want to minimize the number of indexes and ensure queries are covered. Which TWO of the following statements are true regarding the creation of a compound index to achieve these goals? (Choose two.)

Question 20hardmultiple choice

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You have a collection `orders` with a compound index on `{ customer_id: 1, order_date: -1, status: 1 }`. A query filters on `customer_id` and `status`, and sorts by `order_date` descending. Which statement best describes the index's ability to support this query efficiently?

Question 21hardmultiple choice

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A financial application runs a query that filters on `accountId` equality and `balance` range, then sorts by `lastUpdated` descending. The developer creates the index { accountId: 1, balance: 1, lastUpdated: -1 }. During testing, the planner selects this index but the plan shows a SORT stage. Which change should the developer make to eliminate the blocking sort?

Question 22hardmultiple choice

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A developer is designing an index for a collection 'inventory' that will be sharded. The shard key is { warehouseId: 1, itemId: 1 }. They want to create an index that supports queries that filter on warehouseId and sort by quantity. Which index should they create to optimize these queries while being compatible with the sharded cluster?

Question 23mediummulti select

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A developer is designing indexes for a MongoDB collection that stores product data. The collection has a field `tags` that is an array of strings, and a field `price` that is a number. The developer needs to support queries that filter on both `tags` and `price`, and also queries that sort by `price`. Which TWO of the following statements are true regarding the creation of a compound index on `{ tags: 1, price: 1 }`? (Choose two.)

Question 24mediummultiple choice

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A MongoDB collection stores event logs with a 'timestamp' field and a 'level' field. The collection is very large, and most queries filter on level: 'ERROR' and sort by timestamp descending. However, only about 5% of documents have level 'ERROR'. You want to create an index that minimizes index size and improves write performance while still supporting these queries. Which index should you create?

Question 25mediummultiple choice

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You have a collection of user activity logs where queries frequently filter by `userId` and sort by `timestamp` in descending order. Which single index definition provides optimal support for this query pattern without requiring an in-memory sort?

Question 26mediummultiple choice

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An e-commerce application frequently executes a query filtering by 'category' and 'brand', and then sorts the results by 'price' in ascending order. You created a compound index on { category: 1, brand: 1, price: 1 }. Which statement best describes how MongoDB utilizes this index for a query that filters only on 'brand' and 'price'?

Question 27mediummultiple choice

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You are designing a schema for a social media platform where users have an array of 'tags' and an array of 'mentions'. You attempt to create an index on { tags: 1, mentions: 1 }. What is the most likely outcome of this operation?

Question 28mediummultiple choice

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Your application executes the query: db.users.find({ age: { $gt: 30 } }, { age: 1, _id: 0 }). You have an index on { age: 1 }. How does MongoDB process this query?

Question 29easymultiple choice

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You are considering using a hashed index for a 'product_id' field to distribute data across shards. Which of the following is a known limitation of hashed indexes in MongoDB?

Question 30mediummultiple choice

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Refer to the exhibit. If a query is executed as db.collection.find({ "metadata.type": "book" }), how will MongoDB use the created index?

Exhibit

// Document structure
{
  "_id": 1,
  "metadata": { "type": "book", "genre": "fantasy" }
}

// Index creation
db.collection.createIndex({ "metadata": 1 })

Question 31easymultiple choice

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When creating an index on a large collection in a production environment, what is the primary advantage of using the 'background' option (for versions prior to 4.2) or the standard build process in 4.2+?

Question 32hardmultiple choice

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You are optimizing a query: db.orders.find({ status: 'urgent', amount: { $gt: 500 } }).sort({ order_date: -1 }). According to the ESR (Equality, Sort, Range) rule, what is the ideal index for this query?

Question 33mediummultiple choice

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A developer wants to implement full-text search on a collection of articles. They create a text index on the 'content' field. Which of the following is a significant limitation of 'text' indexes in MongoDB?

Question 34mediummultiple choice

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You are storing GPS coordinates in a 'locations' collection using GeoJSON format. You need to perform queries that find points within a specific polygon. Which index type must be created on the location field?

Question 35mediummultiple choice

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You are optimizing a collection containing user logs. Queries frequently filter by 'category' and sort by 'timestamp' in descending order. Which index configuration best supports these queries while maintaining efficiency?

Question 36mediummultiple choice

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You need to ensure that values in the 'email' field are unique across all documents in a collection, but documents where the 'email' field is missing should not violate this constraint. How should you create this index?

Question 37easymultiple choice

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Which index type should be used to support efficient queries on a field containing geographical coordinates in MongoDB?

Question 38mediummulti select

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Which TWO of the following statements regarding the ESR rule for index creation are true?

Question 39hardmultiple choice

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What is the primary consequence of having too many indexes on a collection that experiences a high volume of write operations?

Question 40mediummultiple choice

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Which property of a Multikey Index is true when indexing an array field?

Question 41easymultiple choice

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What is the primary function of the explain() method in MongoDB?

Question 42hardmultiple choice

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What is the primary benefit of using a partial index in MongoDB?

Question 43mediummultiple choice

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When should a TTL (Time-To-Live) index be used in a MongoDB collection?

Question 44hardmultiple choice

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What is the primary technical limitation when creating a covered query?

Question 45mediummultiple choice

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A support team frequently runs a query that filters on status and sorts by created_at, both fields in the same collection. An existing index { status: 1 } is in place, but the query still performs an in-memory sort and gets killed once result sets grow past the 100 MB sort limit. Which index should you create to let the server satisfy both the filter and the sort without a blocking sort stage?

Question 46hardmultiple choice

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An application queries a users collection with db.users.find({ email: "a@b.com" }). The collection holds about 2 million documents, and the email field is unique for every user. A developer proposes creating an index { email: 1, country: 1 } to speed up the query. What is the most accurate assessment of this proposal?

Question 47mediummultiple choice

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A logistics application stores shipment records in a collection where each document has a numeric field `weight_kg`. Queries frequently filter on `weight_kg` with a range (e.g., between 50 and 200) and sort by `weight_kg` ascending. You create a standard ascending index on `weight_kg`. Which statement best describes the index's behavior for these queries?

Question 48mediummultiple choice

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A logistics company stores shipment events in a collection where each document contains a `history` array of subdocuments with fields `status`, `timestamp`, and `location`. Queries frequently filter on `history.status` and sort by `history.timestamp`. An index is created on `history.status`. What is the most accurate statement about this index?

Question 49hardmultiple choice

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A financial application has a collection `transactions` with fields `accountId`, `type`, `amount`, and `createdAt`. A query filters on `accountId` and `type`, sorts descending by `createdAt`, and projects only `_id` and `amount`. A developer creates the index `{ accountId: 1, type: 1, createdAt: -1 }`. Which statement best describes whether this index can support the query as a covered query?

Question 50mediummultiple choice

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A logistics application stores shipment records in a collection named `shipments`. Each document includes a `status` field (e.g., "in_transit", "delivered") and a `delivery_date` field. The development team frequently runs a query that filters on `status` and sorts by `delivery_date` in ascending order. They want to create a compound index to optimize this query. Which index should they create?

Question 51mediummultiple choice

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A retail application queries an `orders` collection with a filter on `status` and `customerId`, and a sort on `orderDate`. The query shape is: `db.orders.find({ status: "shipped", customerId: 12345 }).sort({ orderDate: -1 })`. The collection has no indexes other than the default `_id` index. Which index should be created to best support this query according to the ESR guideline?

Question 52easymultiple choice

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A developer is working with a collection that stores user profiles. The collection has an index on the `email` field. The developer runs a query that uses a regular expression to find all users whose email ends with "@example.com". The query is: `db.users.find({ email: /@example\.com$/ })`. What is the most likely behavior of this query with respect to the index?

Question 53hardmulti select

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A developer is working with a MongoDB collection that stores user profiles. The collection has a unique index on `email` and a compound index on `{ lastName: 1, firstName: 1 }`. The developer needs to enforce that no two documents have the same combination of `tenantId` and `username`. Which TWO of the following statements are correct regarding the creation and behavior of a unique compound index for this requirement? (Choose two.)

Question 54mediummultiple choice

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A developer maintains a collection of sensor readings. Queries almost always filter on `status` and then sort by `timestamp` descending. The current index is { status: 1, timestamp: 1 }. The developer observes that the query planner selects a COLLSCAN even though the index covers both fields. Which index change should be made to allow an efficient index scan for this query?

Question 55mediummultiple choice

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An IoT platform stores sensor readings in a collection `readings` with fields `deviceId`, `timestamp`, and `value`. The collection is large and grows continuously. Queries typically retrieve the most recent readings for a specific device, filtering by `deviceId` and sorting by `timestamp` descending. The operations team wants to minimize index size while still supporting these queries efficiently. Which index strategy best meets this requirement?

Question 56hardmultiple choice

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An application writes documents to a `metrics` collection where each document contains an embedded array `samples` of subdocuments with fields `value` and `unit`. A query filters on `samples.unit` and `samples.value` together, expecting both conditions to match the same array element. The developer creates { "samples.unit": 1, "samples.value": 1 }. Which behavior should the developer expect from this index?

Question 57mediummultiple choice

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A collection 'orders' has a compound index { status: 1, customerId: 1, orderDate: -1 }. A developer runs a query: db.orders.find({ customerId: 'C123', status: 'shipped' }).sort({ orderDate: -1 }). Which statement about how MongoDB uses this index for the query is correct?

Question 58easymultiple choice

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A team stores user profiles in a `profiles` collection. About 90% of documents have a `deletedAt` field set to null, and only 10% have an actual timestamp indicating soft deletion. Queries that list active users filter on `deletedAt: null` and sort by `createdAt`. The team wants an index that stays small and avoids maintaining entries for deleted documents. Which index definition best matches this requirement?

Question 59mediummultiple choice

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You are the lead developer for a high-traffic e-commerce application using MongoDB 6.0. A query on the `orders` collection that filters on `customerId` and sorts by `orderDate` in descending order is performing poorly. You decide to create an index to improve its performance. Which of the following indexes would be most efficient for this query?

Question 60hardmulti select

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A developer is working with a collection 'products' that has an index { category: 1, price: 1 }. They want to understand how MongoDB can use this index for queries that include a sort on price. Which TWO of the following statements are true regarding the use of this index for sorting? (Choose two.)

Question 61mediummultiple choice

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A MongoDB collection contains documents with a field `tags` that is an array of strings. A developer creates an index on the `tags` field. Which of the following statements is true about the resulting index?

Question 62easymultiple choice

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A developer needs to ensure that a `users` collection does not contain two documents with the same email address. Which index type should be created on the `email` field?

Question 63mediummulti select

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A developer is tuning indexes on an `orders` collection. The team reports slow queries and heavy write latency, and they suspect too many indexes. Which TWO practices should the developer apply when reviewing the index set? (Choose two.)

Question 64easymultiple choice

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A developer is working with a MongoDB collection that stores user profiles. The collection has an index on the `email` field. The developer runs a query to find a user by email and then examines the query plan using `explain()`. The output shows that the query used an `IXSCAN` stage. What does this indicate about the query execution?

Question 65hardmultiple choice

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A developer is working with a collection that has a compound index on { a: 1, b: 1, c: 1 }. They run a query that includes a filter on `a` and a sort on `c`. The query is: `db.collection.find({ a: 5 }).sort({ c: 1 })`. Which of the following best describes how MongoDB will execute this query?

Question 66mediummultiple choice

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A collection 'logs' has a TTL index on the 'createdAt' field with expireAfterSeconds set to 3600. A developer notices that some documents older than one hour are still present in the collection. Which explanation is most likely?

Question 67mediummultiple choice

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A logistics application stores shipment records in a MongoDB collection where each document includes a 'status' field (e.g., 'pending', 'in_transit', 'delivered') and a 'createdAt' date. The operations team frequently runs the query: db.shipments.find({ status: 'in_transit' }).sort({ createdAt: -1 }).limit(20). They want an index that allows this query to execute without an in-memory sort and with minimal keys scanned. Which index should you create?

Question 68hardmultiple choice

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You are managing a MongoDB collection that stores sensor data. Each document has a `timestamp` field and a `sensorId` field. The collection is very large, and you need to optimize a query that finds the most recent reading for a specific sensor. The query is: `db.sensors.find({ sensorId: 'abc123' }).sort({ timestamp: -1 }).limit(1)`. Which index would best support this query?

Question 69hardmultiple choice

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A financial analytics collection stores documents with fields 'accountId' (string), 'transactionDate' (date), and 'amount' (decimal). A common query is: db.transactions.find({ accountId: 'A123', transactionDate: { $gte: ISODate('2024-01-01'), $lte: ISODate('2024-01-31') }, amount: { $gt: 1000 } }). You need an index that supports this query efficiently. Following the ESR rule, which index key pattern is optimal?

Question 70mediummulti select

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You are designing indexes for a MongoDB collection that stores user profiles. The collection has a field 'email' that must be unique, and a field 'username' that is also unique. You also need to support queries that find users by a combination of 'country' and 'city'. Which TWO of the following statements are correct regarding index creation and behavior in this scenario? (Choose two.)

Question 71easymultiple choice

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A developer is working with a MongoDB collection that stores sensor readings. Each document has a 'sensorId' field and a 'readings' array containing numeric values. The developer creates an index on the 'readings' field. Which statement best describes the resulting index?

Question 72mediummultiple choice

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A developer is working with a MongoDB collection that has a partial index defined as `db.products.createIndex({ price: 1 }, { partialFilterExpression: { price: { $gt: 100 } } })`. The developer runs a query `db.products.find({ price: { $lt: 50 } })`. Which of the following statements describes how MongoDB will execute this query?

{ "queryPlanner": { "winningPlan": { "stage": "FETCH", "inputStage": { "stage": "IXSCAN", "keyPattern": { "userId": 1, "timestamp": -1 }, "indexName": "userId_1_timestamp_-1", "direction": "forward" } } }, "executionStats": { "nReturned": 50, "totalKeysExamined": 50, "totalDocsExamined": 50 } }