Explain plan analysis (queryPlanner, executionStats) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Understanding queryPlanner output

Given the following MongoDB explain output snippet, what does the queryPlanner section primarily describe?

MongoDB

{
  "queryPlanner": {
    "plannerVersion": 1,
    "namespace": "shop.products",
    "indexFilterSet": false,
    "parsedQuery": { "category": { "$eq": "books" } },
    "winningPlan": {
      "stage": "IXSCAN",
      "keyPattern": { "category": 1 },
      "indexName": "category_1",
      "isMultiKey": false
    }
  },
  "executionStats": { /* omitted for brevity */ }
}

A It contains the error messages if the query failed.

B It shows the actual data returned by the query.

C It describes how MongoDB plans to execute the query, including indexes used.

D It lists all documents scanned during query execution.

💡 Hint

Think about what a 'planner' does before running a query.

The queryPlanner section explains the strategy MongoDB will use to run the query, such as which indexes it will use and the stages involved.

❓ query_result

intermediate

Interpreting executionStats nReturned value

In the executionStats section of a MongoDB explain output, what does the nReturned field represent?

MongoDB

{
  "executionStats": {
    "nReturned": 5,
    "executionTimeMillis": 3,
    "totalKeysExamined": 10,
    "totalDocsExamined": 5
  }
}

A The total number of documents in the collection.

B The number of index keys scanned but not returned.

C The number of queries run on the server.

D The number of documents matched and returned by the query.

💡 Hint

Focus on what 'returned' means in a query context.

nReturned tells how many documents the query actually returned to the user after execution.

🧠 Conceptual

advanced

Difference between queryPlanner and executionStats

Which statement best describes the difference between queryPlanner and executionStats in MongoDB's explain output?

A queryPlanner shows the database schema; executionStats shows user permissions.

B queryPlanner shows the planned steps before running; executionStats shows what actually happened during execution.

C queryPlanner shows server errors; executionStats shows network latency.

D queryPlanner shows the documents returned; executionStats shows the query syntax.

💡 Hint

Think about planning vs doing.

queryPlanner describes the intended plan before running the query, while executionStats reports the real performance and results after running.

📝 Syntax

advanced

Identify the error in explain command usage

Which of the following MongoDB commands will correctly return the explain plan with execution statistics for a find query on collection users filtering by age > 30?

A db.users.find({ age: { $gt: 30 } }).explain('executionStats')

B db.users.explain('executionStats').find({ age: { $gt: 30 } })

C db.users.find.explain({ age: { $gt: 30 } }, 'executionStats')

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

💡 Hint

Remember the order of method calls for explain.

The explain method is called on the cursor returned by find, with the verbosity string as argument.

❓ optimization

expert

Analyzing executionStats for query optimization

You run a query with explain('executionStats') and see totalKeysExamined: 10000 and totalDocsExamined: 10000. What does this suggest about the query's efficiency?

A The query is inefficient because it examines as many documents as index keys, likely scanning many documents.

B The query is efficient because it uses an index and examines few documents.

C The query is efficient because it scanned all documents quickly.

D The query failed because totalDocsExamined cannot equal totalKeysExamined.

💡 Hint

Think about what it means if keys examined equals documents examined.

If totalDocsExamined equals totalKeysExamined and both are large, it means the query scanned many documents, possibly due to poor index usage or no filtering.