Indexes support efficient execution of queries in MongoDB. Without indexes, MongoDB must scan every document in a collection to return query results. MongoDB uses the index to limit the number of documents it must scan.

What are indexes?

Difficulty: 5/10

index creation, query performance, index strategy

In MongoDB, indexes are data structures that store a small portion of the collection's data, enabling efficient query execution by allowing MongoDB to quickly locate documents without scanning the entire collection.

1
Indexes support efficient execution of queries in MongoDB.
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Without indexes, MongoDB must scan every document in a collection to return query results.
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MongoDB uses the index to limit the number of documents it must scan.

Scenario Questions

0-2 years experience

1You need to retrieve all orders for a specific customer ID from a MongoDB collection. How would you use an index to make this query fast, and what command would you run to create it?
2If you run a query that filters on a field that doesn't have an index, what performance impact would you expect and why?

2-5 years experience

1Your team added a compound index on {status:1, createdAt:-1} but notice the query filtering only by status is still slow. What could be causing this, and how would you investigate?

Follow-up Questions

Can you walk me through how you would verify that the index is being used by a query?
What factors would lead you to drop an index you previously added?
How do you monitor index health and fragmentation over time?