MongoDB MongoDB Indexing and Performance 2

Question 1

What is a covered query in MongoDB?

Accepted Answer

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

Answer

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

Question 2

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

Accepted Answer

Multikey index

Answer

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

Question 3

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

Accepted Answer

Automatically deletes documents after a specified time period

Answer

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

Question 4

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

Accepted Answer

Number of documents examined to fulfill the query

Answer

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

Question 5

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

Accepted Answer

Text index

Answer

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

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