Explain plan analysis (queryPlanner, executionStats) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the queryPlanner section in MongoDB's explain plan?

The queryPlanner shows how MongoDB plans to execute a query, including which indexes it will use and the stages of the query execution. It helps understand the strategy MongoDB chooses before running the query.

beginner

What information does the executionStats section provide in MongoDB's explain output?

The executionStats section shows actual runtime details like how many documents were examined, how many were returned, and how long the query took. It helps measure query performance.

intermediate

In MongoDB explain output, what does a high number of totalDocsExamined indicate?

A high totalDocsExamined means MongoDB looked at many documents to find the results. This can suggest the query is inefficient and might benefit from an index.

intermediate

What does the winningPlan represent in the queryPlanner section?

The winningPlan is the chosen method MongoDB will use to run the query, such as which index or collection scan it will perform.

advanced

Why is it useful to compare queryPlanner and executionStats in MongoDB explain?

Comparing queryPlanner and executionStats helps verify if the planned query strategy actually performs well in practice, and if indexes are used effectively.

What does the queryPlanner section in MongoDB explain show?

A The network latency during query execution

B The actual time taken to run the query

C The number of documents returned

D The planned steps and indexes used to execute the query

The queryPlanner shows the planned execution strategy, including indexes and stages.

Which section of MongoDB explain output tells you how many documents were scanned?

A executionStats

B serverStatus

C queryPlanner

D indexStats

The executionStats section reports actual runtime metrics like documents scanned.

If a query's executionStats shows a high totalDocsExamined, what might that mean?

A The query is scanning many documents, possibly inefficient

B The query is using an index efficiently

C The query returned no results

D The query is very fast

High totalDocsExamined means many documents were scanned, which can indicate inefficiency.

What is the winningPlan in MongoDB explain?

A The fastest query result

B The chosen query execution plan

C The number of documents returned

D The server configuration

The winningPlan is the execution plan MongoDB selects to run the query.

Why should you look at both queryPlanner and executionStats?

A To see the server uptime

B To find network errors

C To check if the planned query matches actual performance

D To view user permissions

Comparing both helps confirm if the query plan is efficient in real execution.

Describe what information you find in the queryPlanner and executionStats sections of a MongoDB explain plan.

Think about planning vs actual execution

Explain why analyzing both the queryPlanner and executionStats is important for optimizing MongoDB queries.

Consider planning and real results