Profiler for slow queries in MongoDB

Introduction

The profiler helps find slow database queries so you can make your app faster.

When your app feels slow and you want to see which database queries take the most time.

When you want to check if a new feature causes slow database access.

When you want to improve database performance by fixing slow queries.

When you want to monitor database activity during peak hours.

When you want to learn how your database is used in real time.

Syntax

MongoDB

db.setProfilingLevel(level, { slowms: milliseconds })

// To check current profiling level and slowms
db.getProfilingStatus()

// To see slow queries recorded
db.system.profile.find({ millis: { $gt: threshold } })

level can be 0 (off), 1 (slow queries), or 2 (all queries).

slowms sets the threshold in milliseconds to consider a query slow.

Examples

Turn on profiling for queries slower than 100 milliseconds.

MongoDB

db.setProfilingLevel(1, { slowms: 100 })

Turn off profiling to stop recording queries.

MongoDB

db.setProfilingLevel(0)

Check current profiling level and slow query threshold.

MongoDB

db.getProfilingStatus()

Find and list queries that took more than 200 milliseconds, sorted by slowest first.

MongoDB

db.system.profile.find({ millis: { $gt: 200 } }).sort({ millis: -1 })

Sample Program

This example turns on profiling for queries slower than 50 ms, runs a query, then fetches slow queries from the profiler.

MongoDB

db.setProfilingLevel(1, { slowms: 50 })

// Run a sample slow query
db.orders.find({ price: { $gt: 1000 } }).toArray()

// Check slow queries recorded
const slowQueries = db.system.profile.find({ millis: { $gt: 50 } }).sort({ millis: -1 }).toArray()
slowQueries

Important Notes

Profiling can slow down your database, so use it only when needed.

Remember to turn off profiling (level 0) after you finish to avoid extra load.

The profiler stores data in the system.profile collection inside your database.

Summary

The profiler helps find slow queries by recording them.

Set profiling level and slowms threshold to control what is recorded.

Check system.profile collection to see slow queries and improve performance.