Explain plan analysis (queryPlanner, executionStats) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the explain plan for a MongoDB query.

MongoDB

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

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

AshowPlan

BexplainPlan

Cexplain

DqueryExplain

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not exist like 'explainPlan'.

Trying to use 'showPlan' or 'queryExplain' which are not valid.

The explain() method returns the query plan and execution stats for a MongoDB query.

2fill in blank

medium

Complete the code to get the execution statistics from the explain output.

MongoDB

const result = db.collection.find({ status: 'active' }).explain();
const stats = result.[1];

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

AexecutionStats

BqueryPlanner

CplanSummary

DexecutionPlan

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'queryPlanner' which only shows the plan, not execution details.

Using 'planSummary' which is a brief summary, not detailed stats.

The executionStats field contains detailed information about how the query was executed, including execution time and number of documents examined.

3fill in blank

hard

Fix the error in accessing the winning plan from the explain output.

MongoDB

const plan = db.collection.find({}).explain().[1].winningPlan;

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

AexecutionStats

BplanDetails

CexecutionPlan

DqueryPlanner

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to get winningPlan from 'executionStats' which does not contain it.

Using non-existent fields like 'executionPlan' or 'planDetails'.

The winningPlan is inside the queryPlanner field of the explain output.

4fill in blank

hard

Fill both blanks to get the total number of documents examined and the number returned from execution stats.

MongoDB

const stats = db.collection.find({}).explain().[1];
const examined = stats.[2];

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

AexecutionStats

BqueryPlanner

CtotalDocsExamined

DnReturned

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'queryPlanner' instead of 'executionStats' for runtime data.

Using 'nReturned' instead of 'totalDocsExamined' for documents examined.

The executionStats field contains totalDocsExamined which shows how many documents MongoDB looked at during the query.

5fill in blank

hard

Fill all three blanks to get the number of documents returned, the execution time in milliseconds, and the stage name of the winning plan.

MongoDB

const explain = db.collection.find({}).explain();
const returned = explain.executionStats.[1];
const timeMs = explain.executionStats.[2];
const stage = explain.queryPlanner.winningPlan.[3];

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

AnReturned

BexecutionTimeMillis

Cstage

DtotalDocsExamined

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up fields between executionStats and queryPlanner.

Using 'totalDocsExamined' instead of 'nReturned' for returned documents.

nReturned is the number of documents returned, executionTimeMillis is the time taken, and stage is the name of the winning plan's root stage.

Practice

1. What does the queryPlanner section in MongoDB's explain output show?

easy

A. The plan MongoDB chooses to execute the query

B. The total time taken to run the query

C. The number of documents returned by the query

D. The syntax errors in the query

Solution

1. Step 1: Understand the role of queryPlanner

   The queryPlanner shows the strategy MongoDB picks to run the query, like which indexes it uses.

2. Step 2: Differentiate from executionStats

   executionStats shows runtime details, not the plan itself.

3. Final Answer:

   The plan MongoDB chooses to execute the query -> Option A

4. Quick Check:

   queryPlanner = plan chosen [OK]

Hint: queryPlanner shows the plan, executionStats shows runtime [OK]

Common Mistakes:

• Confusing queryPlanner with executionStats
• Thinking queryPlanner shows query results
• Assuming queryPlanner shows errors

2. Which of the following is the correct way to get execution stats for a query in MongoDB?

easy

A. db.collection.find({}).explain('runStats')

B. db.collection.find({}).explain('queryPlan')

C. db.collection.find({}).explain('statsExecution')

D. db.collection.find({}).explain('executionStats')

Solution

1. Step 1: Recall the correct explain verbosity

   MongoDB uses 'executionStats' to show runtime details.

2. Step 2: Check option syntax

   Only db.collection.find({}).explain('executionStats') uses the correct string 'executionStats' with explain().

3. Final Answer:

   db.collection.find({}).explain('executionStats') -> Option D

4. Quick Check:

   Use 'executionStats' for runtime info [OK]

Hint: Use 'executionStats' string inside explain() for runtime stats [OK]

Common Mistakes:

• Using wrong strings like 'queryPlan' or 'runStats'
• Forgetting quotes around verbosity string
• Confusing method chaining order

3. Given this explain output snippet:

{
  "executionStats": {
    "nReturned": 5,
    "totalDocsExamined": 100,
    "executionTimeMillis": 12
  }
}

What does totalDocsExamined tell you?

medium

A. Number of documents returned by the query

B. Number of documents scanned to find matches

C. Total documents in the collection

D. Number of indexes used

Solution

1. Step 1: Understand totalDocsExamined meaning

   This value shows how many documents MongoDB looked at to find matching results.

2. Step 2: Differentiate from nReturned

   nReturned is how many documents matched and were returned, not scanned.

3. Final Answer:

   Number of documents scanned to find matches -> Option B

4. Quick Check:

   totalDocsExamined = docs scanned [OK]

Hint: totalDocsExamined counts scanned docs, not returned [OK]

Common Mistakes:

• Confusing scanned docs with returned docs
• Thinking it counts indexes used
• Assuming it is total collection size

4. You run db.users.find({age: 30}).explain('executionStat') but get an error. Which is the likely cause?

medium

A. The verbosity string 'executionStats' is misspelled

B. Explain does not support 'executionStats' verbosity

C. The query filter syntax is invalid

D. The collection name is incorrect

Solution

1. Step 1: Check common explain errors

   Misspelling the verbosity string causes errors because MongoDB expects exact strings.

2. Step 2: Validate other options

   Collection name and query syntax are valid; 'executionStats' is supported.

3. Final Answer:

   The verbosity string 'executionStats' is misspelled -> Option A

4. Quick Check:

   Correct spelling of verbosity string is required [OK]

Hint: Check spelling of verbosity string in explain() [OK]

Common Mistakes:

• Misspelling 'executionStats' as 'executionStat'
• Assuming explain() doesn't support 'executionStats'
• Blaming query syntax without checking verbosity

5. You want to optimize a query that scans 1000 documents but returns only 2. The explain output shows totalDocsExamined: 1000 and nReturned: 2. What is the best next step?

hard

A. Rewrite the query to return all documents in the collection

B. Increase the batch size to return more documents faster

C. Add an index on the query fields to reduce scanned documents

D. Ignore the explain output; it does not affect performance

Solution

1. Step 1: Analyze the explain stats

   High totalDocsExamined but low nReturned means many documents scanned unnecessarily.

2. Step 2: Choose optimization strategy

   Adding an index on query fields helps MongoDB scan fewer documents and improve speed.

3. Final Answer:

   Add an index on the query fields to reduce scanned documents -> Option C

4. Quick Check:

   Index reduces scanned docs, improves query [OK]

Hint: High scanned vs returned? Add index on query fields [OK]

Common Mistakes:

• Ignoring explain output and performance impact
• Increasing batch size without fixing scan issue
• Trying to return all documents unnecessarily