Profiler for slow queries in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find slow queries using MongoDB profiler

You have enabled the MongoDB profiler to log slow queries taking longer than 100 milliseconds. Which query will return all slow queries recorded in the profiler?

MongoDB

db.system.profile.find({ millis: { $gt: 100 } })

A db.system.profile.find({ millis: { $gt: 100 } })

B db.system.profile.find({ duration: { $gt: 100 } })

C db.system.profile.find({ time: { $gte: 100 } })

D db.system.profile.find({ executionTimeMillis: { $gt: 100 } })

💡 Hint

The profiler stores the execution time of queries in the 'millis' field.

The MongoDB profiler records the execution time of operations in the 'millis' field. To find slow queries, you filter documents where 'millis' is greater than the threshold.

🧠 Conceptual

intermediate

Profiler levels in MongoDB

Which profiler level setting in MongoDB logs only slow operations exceeding the configured threshold?

A Level 0 - No profiling

B Level 3 - Logs errors only

C Level 1 - Logs slow operations only

D Level 2 - Logs all operations

💡 Hint

Profiler levels control how much detail is recorded.

Level 1 enables profiling only for operations slower than the slowms threshold. Level 2 logs all operations. Level 0 disables profiling.

📝 Syntax

advanced

Enable profiler for slow queries over 200ms

Which command correctly enables the MongoDB profiler to log queries slower than 200 milliseconds?

MongoDB

db.setProfilingLevel(1, { slowms: 200 })

A db.setProfilingLevel({ level: 1, slowms: 200 })

B db.setProfilingLevel(2, { slowms: 200 })

C db.setProfilingLevel(1, 200)

D db.setProfilingLevel(1, { slowms: 200 })

💡 Hint

The first argument is the profiling level, the second is an options object.

The correct syntax is db.setProfilingLevel(level, options). Level 1 enables slow operation profiling with the slowms threshold set in options.

🔧 Debug

advanced

Why does this profiler query return no results?

You run this query to find slow queries: db.system.profile.find({ millis: { $gt: 50 } }).sort({ ts: -1 }).limit(5). But it returns no documents, even though you know slow queries occurred. What is the most likely reason?

A The query syntax is invalid and causes an error

B Profiler is not enabled or set to level 0

C The 'millis' field does not exist in profiler documents

D The sort order should be ascending (1) instead of descending (-1)

💡 Hint

Profiler must be enabled to collect data.

If the profiler is disabled (level 0), no profiling data is collected, so the query returns no results.

❓ optimization

expert

Optimize slow query detection with profiler

You want to detect slow queries but minimize performance impact. Which profiler setting balances detailed slow query logging with minimal overhead?

A Set profiler level to 1 with a high slowms threshold (e.g., 500ms)

B Set profiler level to 2 to log all operations

C Set profiler level to 0 and rely on logs only

D Set profiler level to 1 with slowms set to 1ms

💡 Hint

Higher slowms reduces profiling frequency and overhead.

Level 1 with a high slowms threshold logs only very slow queries, reducing overhead compared to logging all operations or very low thresholds.