Explain plan analysis (queryPlanner, executionStats) in MongoDB

Introduction

Explain plan analysis helps you understand how MongoDB runs your query. It shows the steps MongoDB takes to find your data and how long it takes.

You want to see if your query is using indexes to be faster.

You need to find why a query is slow and how to improve it.

You want to check how many documents MongoDB scans to answer your query.

You want to compare different query methods to pick the best one.

Syntax

MongoDB

db.collection.find(query).explain('executionStats')

queryPlanner shows the plan MongoDB chooses to run your query.

executionStats adds details about how many documents were scanned and how long it took.

Examples

Shows the query plan without execution details.

MongoDB

db.users.find({ age: { $gt: 25 } }).explain('queryPlanner')

Shows the query plan plus how many documents were scanned and time taken.

MongoDB

db.users.find({ age: { $gt: 25 } }).explain('executionStats')

Defaults to queryPlanner mode if no argument is given.

MongoDB

db.users.find({ name: 'Alice' }).explain()

Sample Program

This inserts three users, then runs a query to find users older than 25. It shows the query plan and execution stats.

MongoDB

db.users.insertMany([
  { name: 'Alice', age: 30 },
  { name: 'Bob', age: 20 },
  { name: 'Carol', age: 35 }
])

const result = db.users.find({ age: { $gt: 25 } }).explain('executionStats')
printjson(result)

Important Notes

If your query does not use an index, MongoDB will do a collection scan (COLLSCAN), which is slower.

Look at totalDocsExamined vs nReturned to see efficiency.

Use explain('executionStats') to get detailed timing and document counts.

Summary

Explain plan shows how MongoDB runs your query step-by-step.

Use queryPlanner to see the plan chosen.

Use executionStats to see how many documents were scanned and how long it took.