MongoDB Indexing and Performance Flashcards

What is a covered query in MongoDB?

Answer: A query that can be satisfied entirely using an index without accessing documents

A covered query is fulfilled entirely by the index, meaning MongoDB never needs to read actual documents, resulting in very high performance.

Which type of index supports efficient queries on fields within embedded arrays?

Answer: Multikey index

MongoDB automatically creates a multikey index when an indexed field contains array values, creating index entries for each array element.

What is the purpose of a TTL (Time-To-Live) index in MongoDB?

Answer: Automatically deletes documents after a specified time period

A TTL index automatically removes documents from a collection after a specified number of seconds, useful for session data or logs.

What does the 'nscannedObjects' field in explain() output indicate?

Answer: Number of documents examined to fulfill the query

nscannedObjects (or docsExamined in newer versions) shows how many actual documents were read, with lower numbers indicating better index usage.

Which index type allows full-text search queries on string content?

Answer: Text index

A text index tokenizes and stems string fields to support $text search queries, with at most one text index per collection.

What is a partial index in MongoDB?

Answer: An index that only includes documents meeting a specified filter expression

A partial index indexes only documents that satisfy a specified filter, resulting in smaller, more efficient indexes for common query patterns.