Profiler for slow queries in MongoDB - Mini Project: Build & Apply

Profiler for Slow Queries in MongoDB

📖 Scenario: You are a database administrator for a small online store. You want to find out which queries are running slowly in your MongoDB database so you can improve performance.

🎯 Goal: Build a simple MongoDB profiler setup that records slow queries and then query the profiler data to find those slow queries.

📋 What You'll Learn

Enable the MongoDB profiler to log slow queries

Set a threshold for what counts as a slow query

Query the profiler collection to find slow queries

Limit the output to the slowest queries

💡 Why This Matters

🌍 Real World

Database administrators use profiling to identify slow queries that affect application performance and user experience.

💼 Career

Knowing how to use the MongoDB profiler is important for roles like DBA, backend developer, and performance engineer.

1

Enable the MongoDB profiler

Enable the MongoDB profiler on the current database with level 1 to log slow queries.

MongoDB

// Enable the profiler with level 1
// Your code here

Hint

Use db.setProfilingLevel(1) to enable profiling of slow queries.

2

Set the slow query threshold

Set the slow query threshold to 100 milliseconds using slowms option in the profiler configuration.

MongoDB

db.setProfilingLevel(1)
// Set slowms to 100 milliseconds
// Your code here

Hint

Use db.setProfilingLevel(1, { slowms: 100 }) to set the threshold.

3

Query the profiler for slow queries

Write a query on the system.profile collection to find all queries where millis is greater than 100.

MongoDB

db.setProfilingLevel(1, { slowms: 100 })
// Query system.profile for slow queries
// Your code here

Hint

Use db.system.profile.find({ millis: { $gt: 100 } }) to find slow queries.

4

Sort and limit the slow queries output

Modify the query to sort the slow queries by millis in descending order and limit the output to 5 documents.

MongoDB

db.setProfilingLevel(1, { slowms: 100 })
// Sort by millis descending and limit to 5
// Your code here

Hint

Use .sort({ millis: -1 }) and .limit(5) on the query.

Practice

1. What is the main purpose of the MongoDB profiler when set to capture slow queries?

easy

A. To increase the speed of all queries by caching results

B. To record queries that take longer than a specified time to help optimize performance

C. To delete slow queries from the database

D. To automatically fix slow queries without user intervention

Solution

1. Step 1: Understand profiler's role

   The profiler records queries that exceed a time threshold to help identify slow operations.

2. Step 2: Identify what profiler does not do

   It does not fix or delete queries automatically; it only logs them for review.

3. Final Answer:

   To record queries that take longer than a specified time to help optimize performance -> Option B

4. Quick Check:

   Profiler logs slow queries = To record queries that take longer than a specified time to help optimize performance [OK]

Hint: Profiler logs slow queries for review, not automatic fixes [OK]

Common Mistakes:

• Thinking profiler fixes queries automatically
• Confusing profiler with query caching
• Assuming profiler deletes slow queries

2. Which command correctly sets the MongoDB profiler to log all queries slower than 100 milliseconds?

easy

A. db.setProfilingLevel(2, { slowms: 100 })

B. db.setProfilingLevel(0, { slowms: 100 })

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

D. db.setProfilingLevel(1, 100)

Solution

1. Step 1: Recall profiling levels

   Level 1 logs slow queries exceeding the slowms threshold.

2. Step 2: Check correct syntax

   The correct syntax uses level and options object with slowms key, so db.setProfilingLevel(1, { slowms: 100 }) is valid.

3. Final Answer:

   db.setProfilingLevel(1, { slowms: 100 }) -> Option C

4. Quick Check:

   Level 1 + slowms option = db.setProfilingLevel(1, { slowms: 100 }) [OK]

Hint: Use level 1 with slowms in options object for slow query logging [OK]

Common Mistakes:

• Using level 2 to log only slow queries (it logs all queries)
• Passing slowms as second argument without object
• Setting level 0 which disables profiling

3. Given the following profiler query:

db.system.profile.find({ millis: { $gt: 50 } })

What does this query return?

medium

A. All queries that took more than 50 milliseconds

B. All queries that took less than 50 milliseconds

C. All queries that took exactly 50 milliseconds

D. All queries regardless of execution time

Solution

1. Step 1: Understand the query filter

   The filter { millis: { $gt: 50 } } selects documents where the millis field is greater than 50.

2. Step 2: Interpret millis field meaning

   Millis records query execution time in milliseconds, so this finds queries slower than 50ms.

3. Final Answer:

   All queries that took more than 50 milliseconds -> Option A

4. Quick Check:

   Filter $gt 50 means queries > 50ms = All queries that took more than 50 milliseconds [OK]

Hint: $gt means greater than, so queries slower than threshold [OK]

Common Mistakes:

• Confusing $gt with $lt (greater than vs less than)
• Thinking it returns queries equal to 50ms
• Assuming it returns all queries

4. You run db.setProfilingLevel(1, { slowms: '100' }) but no slow queries are logged. What is the problem?

medium

A. The profiler only logs queries slower than 1000ms by default

B. Profiling level 1 does not log slow queries

C. You must restart MongoDB after setting profiling level

D. The slowms value should be a number, not a string

Solution

1. Step 1: Check slowms parameter type

   slowms expects a number, but '100' is a string, causing it to ignore the threshold.

2. Step 2: Understand profiling behavior

   Profiling level 1 logs slow queries, no restart needed, and default slowms is 100ms if not set.

3. Final Answer:

   The slowms value should be a number, not a string -> Option D

4. Quick Check:

   slowms must be numeric, not string = The slowms value should be a number, not a string [OK]

Hint: Use number type for slowms, not string quotes [OK]

Common Mistakes:

• Passing slowms as string instead of number
• Thinking profiling level 1 disables logging
• Believing MongoDB needs restart after setting profiler

5. You want to find the top 3 slowest queries recorded by the profiler. Which query will correctly return these?

hard

A. db.system.profile.find().sort({ millis: -1 }).limit(3)

B. db.system.profile.find().sort({ millis: 1 }).limit(3)

C. db.system.profile.find({ millis: { $lt: 3 } })

D. db.system.profile.find().limit(3)

Solution

1. Step 1: Sort queries by execution time descending

   Sorting by millis: -1 orders queries from slowest to fastest.

2. Step 2: Limit results to top 3

   Using limit(3) returns only the first three documents after sorting.

3. Final Answer:

   db.system.profile.find().sort({ millis: -1 }).limit(3) -> Option A

4. Quick Check:

   Sort descending + limit 3 = db.system.profile.find().sort({ millis: -1 }).limit(3) [OK]

Hint: Sort by millis descending, then limit 3 for slowest queries [OK]

Common Mistakes:

• Sorting millis ascending returns fastest queries
• Filtering millis less than 3 returns wrong data
• Not sorting before limiting results