Profiler for slow queries in MongoDB - Step-by-Step Execution

Concept Flow - Profiler for slow queries

Enable Profiler

↓

Profiler Logs Queries

↓

Check Query Execution Time

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Log Query

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Analyze Logs

The profiler is turned on, it watches queries, logs those that are slow, and then you analyze these logs to improve performance.

Execution Sample

MongoDB

db.setProfilingLevel(1, { slowms: 100 })
db.system.profile.find({ millis: { $gt: 100 } })

Enable profiling for queries slower than 100ms and then find those slow queries in the profiler collection.

Execution Table

Step	Action	Profiler State	Query Logged	Output
1	Enable profiler with slowms=100	Profiling ON, slowms=100	No	Profiler ready
2	Run query taking 50ms	Profiling ON	No	Query runs, not logged (50 < 100)
3	Run query taking 150ms	Profiling ON	Yes	Query logged (150 > 100)
4	Query profiler collection for slow queries	Profiling ON	N/A	Returns logged slow query details
5	Disable profiler	Profiling OFF	No	Profiling stopped

💡 Profiler stops logging when disabled or slowms threshold not met

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
profilingLevel	0 (off)	1 (on)	1 (on)	1 (on)	1 (on)	0 (off)
slowms	100 (default)	100	100	100	100	100
loggedQueriesCount	0	0	0	1	1	1

Key Moments - 2 Insights

Why doesn't every query get logged when the profiler is on?

Only queries slower than the slowms threshold are logged, as shown in execution_table step 2 and 3 where 50ms query is ignored but 150ms query is logged.

What happens if you disable the profiler?

Profiler stops logging any queries immediately, as shown in execution_table step 5 where profilingLevel changes to 0 and no queries are logged.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the first slow query logged?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Query Logged' column in execution_table rows

According to variable_tracker, what is the profilingLevel after enabling the profiler?

A 0 (off)

B 2 (debug)

C 1 (on)

D 100

💡 Hint

Look at profilingLevel value after Step 1 in variable_tracker

If slowms was set to 200, what would happen to the query taking 150ms in step 3?

A It would not be logged

B Profiler would disable

C It would be logged

D Query would run faster

💡 Hint

Compare query time with slowms threshold in execution_table step 3

Concept Snapshot

Profiler for slow queries in MongoDB:
- Enable with db.setProfilingLevel(level, {slowms: ms})
- Logs queries slower than slowms
- Check logs in system.profile collection
- Disable with db.setProfilingLevel(0)
- Helps find slow queries to optimize

Full Transcript

This visual execution shows how MongoDB's profiler works for slow queries. First, you enable the profiler with a threshold (slowms) that sets the minimum query time to log. Queries faster than this threshold are ignored. When a query runs slower than slowms, it is logged in the system.profile collection. You can then query this collection to see details of slow queries. Finally, you can disable the profiler to stop logging. Variables like profilingLevel and slowms control this behavior, and the loggedQueriesCount tracks how many slow queries have been recorded.

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