explain method for query analysis in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to analyze a MongoDB query using the explain method.

MongoDB

db.collection.find({ name: 'Alice' }).[1]()

Drag options to blanks, or click blank then click option'

Aexplain

Bexecute

Crun

Danalyze

Attempts:

3 left

💡 Hint

Common Mistakes

Using run() or execute() instead of explain()

Forgetting to call the method with parentheses

The explain() method shows how MongoDB runs a query, helping you understand its performance.

2fill in blank

medium

Complete the code to get the execution stats of a query using explain.

MongoDB

db.collection.find({ age: { $gt: 25 } }).explain('[1]')

Drag options to blanks, or click blank then click option'

Aperformance

Bdetailed

Csummary

DexecutionStats

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'detailed' or 'summary' which are not valid verbosity options

Passing no argument or wrong string

The executionStats verbosity level provides detailed information about the query execution.

3fill in blank

hard

Fix the error in the code to correctly use explain with verbosity.

MongoDB

db.collection.find().explain([1])

Drag options to blanks, or click blank then click option'

AexecutionStats

B'executionStats'

C'detailed'

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the verbosity without quotes causing syntax error

Using invalid verbosity values

The verbosity argument must be a string inside quotes, like 'executionStats'.

4fill in blank

hard

Fill both blanks to explain a query with 'allPlansExecution' verbosity and filter by status 'active'.

MongoDB

db.users.find({ status: '[1]' }).explain('[2]')

Drag options to blanks, or click blank then click option'

Aactive

Binactive

CallPlansExecution

DexecutionStats

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong status value

Using wrong verbosity string

Use 'active' to filter users and 'allPlansExecution' to get detailed plan info.

5fill in blank

hard

Fill all three blanks to explain a query on 'products' collection filtering price > 100 with 'queryPlanner' verbosity.

MongoDB

db.[1].find({ price: { $[2]: 100 } }).explain('[3]')

Drag options to blanks, or click blank then click option'

Aproducts

Bgt

CqueryPlanner

Ditems

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong collection name

Using wrong operator like 'lt' or 'gte'

Using wrong verbosity string

The collection is products, the operator is gt for greater than, and verbosity queryPlanner shows the plan.

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