61. Which command creates a time-series collection?
- a) db.createCollection("logs", { timeseries: { timeField: "timestamp" } })
- b) db.createTimeseries()
- c) db.makeTimeSeries()
- d) db.collection.timeseries()
Answer: A - Time-series collections optimize sequential time-based data storage.
62. What is the purpose of the $densify operator?
- a) Adds missing values in time-series data
- b) Compresses documents
- c) Encrypts field values
- d) Normalizes data
Answer: A - $densify inserts documents if values are missing in a range (e.g., timestamps).
63. Which method returns execution statistics for a query?
- a) explain("executionStats")
- b) stats()
- c) analyze()
- d) executionInfo()
Answer: A - db.collection.find().explain("executionStats") shows performance metrics.
64. What does the $rand operator do?
- a) Generates random float between 0 and 1
- b) Selects random documents
- c) Shuffles array elements
- d) Creates random strings
Answer: A - { $rand: {} } produces a pseudo-random number in aggregation.
65. Which command starts a MongoDB transaction?
- a) session.startTransaction()
- b) db.beginTransaction()
- c) transaction.start()
- d) start.transaction()
Answer: A - Transactions require a session: const session = db.getMongo().startSession().
66. What is the purpose of the $accumulator operator?
- a) Custom aggregation logic with JavaScript
- b) Sums values automatically
- c) Counts documents
- d) Accumulates system stats
Answer: A - $accumulator allows custom aggregation functions (e.g., weighted averages).
67. Which operator validates documents against a JSON Schema?
- a) $jsonSchema
- b) $validate
- c) $matchSchema
- d) $checkSchema
Answer: A - { $jsonSchema: { ... } } enforces document structure during queries.
68. What does the $linearFill operator do?
- a) Fills nulls using linear interpolation
- b) Draws linear graphs
- c) Creates linear indexes
- d) Optimizes linear searches
Answer: A - $linearFill computes missing values between known points in time-series data.
69. Which method creates a search index?
- a) db.collection.createSearchIndex()
- b) db.collection.searchIndex()
- c) db.createSearchIndex()
- d) makeSearchIndex()
Answer: A - Search indexes power MongoDB Atlas full-text search capabilities.
70. What is the purpose of the $unionWith stage?
- a) Combines results from multiple collections
- b) Joins collections permanently
- c) Merges schemas
- d) Unifies indexes
Answer: A - $unionWith performs a union of pipelines from different collections.
71. Which operator rounds numbers to integer?
- a) $round
- b) $trunc
- c) $floor
- d) All of the above
Answer: D - All these operators convert to integers with different rounding behaviors.
72. What does the $getField operator do?
- a) Dynamically accesses fields using string names
- b) Gets field metadata
- c) Validates field existence
- d) Extracts subfields
Answer: A - { $getField: { field: "name", input: "$object" } } accesses nested fields.
73. Which command changes stream settings?
- a) db.adminCommand({ setChangeStream: true })
- b) db.enableChangeStream()
- c) mongod --changeStream
- d) db.setStreaming()
Answer: A - Change streams require configuration at database or deployment level.
74. What is the purpose of the $tsIncrement operator?
- a) Extracts increment value from timestamp
- b) Creates incremental timestamps
- c) Auto-increments fields
- d) Measures time intervals
Answer: A - $tsIncrement gets the increment portion of a MongoDB timestamp (for change streams).
75. Which method returns change stream events?
- a) db.collection.watch()
- b) db.collection.changes()
- c) db.getChanges()
- d) db.stream()
Answer: A - watch() returns a cursor for real-time data changes (inserts/updates/deletes).
76. What does the $binary operator represent?
- a) Binary data (BSON BinData type)
- b) Binary comparisons
- c) Binary logs
- d) Binary indexes
Answer: A - { $binary: "base64data", $type: "00" } stores binary payloads.
77. Which operator implements bitwise operations?
- a) $bitsAllSet
- b) $bit
- c) $bitsAnyClear
- d) All of the above
Answer: D - MongoDB provides multiple bitwise query operators.
78. What is the purpose of the $meta operator?
- a) Accesses metadata like text search scores
- b) Defines document metadata
- c) Stores schema information
- d) Validates document structure
Answer: A - { $meta: "textScore" } retrieves computed values like relevance scores.
79. Which command enables client-side field-level encryption?
- a) mongod --enableEncryption
- b) db.enableFieldEncryption()
- c) Requires driver configuration
- d) db.encryptFields()
Answer: C - Field-level encryption is configured in the MongoDB driver/client.
80. What does the $function operator allow?
- a) Custom JavaScript in aggregations
- b) Function indexing
- c) Stored procedures
- d) Trigger definitions
Answer: A - $function executes custom JavaScript functions in aggregation pipelines (requires server-side scripting enabled).