explain method for query analysis in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Understanding explain() output stages

You run the following MongoDB query with explain():

db.orders.find({status: 'shipped'}).explain('executionStats')

Which stage in the explain output shows the number of documents examined?

MongoDB

db.orders.find({status: 'shipped'}).explain('executionStats')

A executionStats.totalDocsExamined

B executionStats.nReturned

C queryPlanner.winningPlan.inputStage.nReturned

D executionStats.executionTimeMillis

💡 Hint

Look for the field that counts documents scanned during query execution.

The field executionStats.totalDocsExamined shows how many documents MongoDB scanned to fulfill the query.

🧠 Conceptual

intermediate

Purpose of the explain() method in MongoDB

What is the main purpose of using the explain() method when running a MongoDB query?

A To create an index automatically for the query

B To show the query execution plan and performance details

C To modify documents matching the query

D To delete documents matching the query

💡 Hint

Think about what information helps you understand how MongoDB runs your query.

The explain() method reveals how MongoDB executes a query, including indexes used and performance metrics.

📝 Syntax

advanced

Correct syntax for explain() with verbosity mode

Which of the following is the correct syntax to run explain() on a find query with 'allPlansExecution' verbosity in MongoDB?

A db.collection.find({age: {$gt: 30}}).explain('allPlansExecution')

B db.collection.explain('allPlansExecution').find({age: {$gt: 30}})

C db.collection.find({age: {$gt: 30}}).explain.allPlansExecution()

D db.collection.find.explain('allPlansExecution')({age: {$gt: 30}})

💡 Hint

explain() is called after the find() method with the verbosity string as argument.

The explain() method is called on the cursor returned by find(), passing the verbosity mode as a string argument.

❓ optimization

advanced

Using explain() to identify slow queries

You notice a query is slow. How can explain() help you find the cause?

A By changing the query to use a faster operator

B By automatically speeding up the query execution

C By deleting unnecessary documents to improve speed

D By showing if the query is scanning many documents without using an index

💡 Hint

Think about what explain() reveals about how MongoDB processes queries.

Explain() shows if the query uses indexes or scans many documents, which can cause slowness.

🔧 Debug

expert

Diagnosing an error in explain() usage

You run this command:

db.users.explain('executionStats').find({active: true})

But it throws an error. What is the cause?

MongoDB

db.users.explain('executionStats').find({active: true})

A explain() cannot be used with find() queries

B The verbosity string 'executionStats' is invalid

C explain() must be called after find(), not before

D The query object {active: true} is malformed

💡 Hint

Check the order of method calls in MongoDB query syntax.

explain() is a method on the cursor returned by find(), so it must be called after find().