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().

Practice

1. What is the main purpose of the explain method in MongoDB?

easy

A. To show how MongoDB executes a query and if indexes are used

B. To delete documents matching a query

C. To update documents in a collection

D. To create a new index on a collection

Solution

1. Step 1: Understand the role of explain

   The explain method is used to analyze how MongoDB runs a query, including index usage and document scanning.

2. Step 2: Compare with other options

   Deleting, updating, or creating indexes are different operations and not related to query analysis.

3. Final Answer:

   To show how MongoDB executes a query and if indexes are used -> Option A

4. Quick Check:

   explain shows query execution details = C [OK]

Hint: explain shows query plan and index use [OK]

Common Mistakes:

• Confusing explain with update or delete operations
• Thinking explain modifies data
• Assuming explain creates indexes

2. Which of the following is the correct syntax to use the explain method on a find query in MongoDB?

easy

A. db.collection.find().explain()

B. db.collection.explain.find()

C. db.collection.find.explain()

D. db.explain.collection.find()

Solution

1. Step 1: Recall the correct method chaining

   In MongoDB, explain() is called after the query method, so db.collection.find().explain() is correct.

2. Step 2: Identify incorrect chaining

   Options A, C, and D incorrectly place explain before or inside the query method, which is invalid syntax.

3. Final Answer:

   db.collection.find().explain() -> Option A

4. Quick Check:

   explain called after find() = B [OK]

Hint: Call explain() after the query method like find() [OK]

Common Mistakes:

• Placing explain before find()
• Using explain as a collection method
• Missing parentheses on explain()

3. Given the query db.users.find({ age: { $gt: 30 } }).explain(), what information does the output primarily provide?

medium

A. The number of documents with age greater than 30

B. How MongoDB searched documents and if an index was used

C. The exact documents returned by the query

D. The total size of the users collection

Solution

1. Step 1: Understand what explain outputs

   The explain() method shows the query plan, including whether indexes were used and how many documents were scanned.

2. Step 2: Differentiate from query results

   It does not return the actual documents or collection size, only query execution details.

3. Final Answer:

   How MongoDB searched documents and if an index was used -> Option B

4. Quick Check:

   explain shows query plan, not data = A [OK]

Hint: explain shows query plan details, not actual data [OK]

Common Mistakes:

• Expecting explain to return documents
• Confusing explain output with collection stats
• Thinking explain counts matching documents

4. You run db.orders.find({ status: 'shipped' }).explain but get an error. Which of the following is the most likely cause?

medium

A. The explain method must be called on the collection, not the query

B. The collection orders does not exist

C. The query filter syntax is invalid

D. You forgot parentheses after explain, it should be explain()

Solution

1. Step 1: Check syntax of explain usage

   The explain method requires parentheses to be called as a function: explain().

2. Step 2: Verify other options

   Calling explain on the query is correct, the filter syntax is valid, and collection existence would cause a different error.

3. Final Answer:

   You forgot parentheses after explain, it should be explain() -> Option D

4. Quick Check:

   explain needs () to run = D [OK]

Hint: Always use explain() with parentheses [OK]

Common Mistakes:

• Calling explain without ()
• Thinking explain is a collection method
• Assuming query filter is wrong without checking

5. You want to optimize a slow query on the products collection. After running db.products.find({ category: 'books' }).explain('executionStats'), you see a high number of documents examined but few returned. What should you do next?

hard

A. Rewrite the query to use aggregate() instead of find()

B. Increase the batch size of the query to reduce network calls

C. Create an index on the category field to speed up filtering

D. Drop the products collection and recreate it

Solution

1. Step 1: Analyze explain output for optimization clues

   A high number of documents examined but few returned means MongoDB is scanning many documents without using an index efficiently.

2. Step 2: Choose the best optimization action

   Creating an index on the category field helps MongoDB quickly find matching documents, reducing scanned documents and speeding up the query.

3. Final Answer:

   Create an index on the category field to speed up filtering -> Option C

4. Quick Check:

   High docs examined means add index = A [OK]

Hint: High docs examined? Add index on filter field [OK]

Common Mistakes:

• Ignoring index creation for slow queries
• Trying unrelated query changes without index
• Dropping collection unnecessarily