Index usage verification in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using indexes in MongoDB?

Indexes help MongoDB find data faster by creating a quick lookup for fields, similar to an index in a book.

beginner

Which MongoDB command shows how a query uses indexes?

The explain() method shows details about how MongoDB executes a query and which indexes it uses.

intermediate

What does the winningPlan field in explain() output indicate?

It shows the best plan MongoDB chose to run the query, including which index it used if any.

intermediate

How can you verify if a query is using a collection scan instead of an index?

If explain() shows COLLSCAN in the stage, it means MongoDB scanned the whole collection without using an index.

advanced

Why might MongoDB choose not to use an index even if one exists?

MongoDB may skip an index if it thinks scanning the whole collection is faster, for example if the query returns many documents.

Which MongoDB method helps you check if a query uses an index?

A explain()

B findIndex()

C showIndexes()

D listIndexes()

The explain() method shows how MongoDB runs a query and which indexes it uses.

What does a COLLSCAN stage in explain() output mean?

A Query used a cached result

B Query used an index

C Query failed

D Query scanned the entire collection

COLLSCAN means MongoDB scanned all documents without using an index.

If a query returns many documents, MongoDB might:

A Always use an index

B Throw an error

C Skip the index and scan the collection

D Create a new index automatically

MongoDB may skip indexes if scanning the collection is faster for large result sets.

Which field in explain() output shows the chosen query plan?

A queryPlanner.winningPlan

B executionStats.totalDocsExamined

C serverStatus

D indexStats

queryPlanner.winningPlan shows the plan MongoDB picked to run the query.

To see all indexes on a MongoDB collection, you use:

A db.collection.getIndexes()

B db.collection.listIndexes()

C db.collection.showIndexes()

D db.collection.explain()

listIndexes() lists all indexes on a collection.

Explain how to verify if a MongoDB query uses an index and what to look for in the explain output.

Think about the query plan and stages shown by explain()

Describe why MongoDB might choose not to use an index even if one exists for a query.

Consider query performance and optimizer decisions