Profiler for slow queries in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable the profiler for slow queries in MongoDB.

MongoDB

db.setProfilingLevel([1])

Drag options to blanks, or click blank then click option'

A2

B0

C3

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using level 0 disables profiling.

Level 3 is not a valid profiling level.

Setting profiling level to 1 enables profiling for slow operations only.

2fill in blank

medium

Complete the code to set the slow operation threshold to 100 milliseconds.

MongoDB

db.setProfilingLevel(1, { slowms: [1] })

Drag options to blanks, or click blank then click option'

A100

B10

C1000

D10000

Attempts:

3 left

💡 Hint

Common Mistakes

Setting slowms too high misses slow queries.

Setting slowms too low may log too many queries.

Setting slowms to 100 means queries slower than 100 milliseconds are profiled.

3fill in blank

hard

Fix the error in the command to retrieve slow queries from the system profiler collection.

MongoDB

db.system.profile.find({ [1]: { $gt: 100 } })

Drag options to blanks, or click blank then click option'

AexecutionTime

BtimeMillis

Cmillis

DdurationMillis

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect field names like 'durationMillis' or 'timeMillis'.

Using 'executionTime' which is not a valid field.

The field 'millis' stores the query duration in milliseconds in the profiler collection.

4fill in blank

hard

Fill both blanks to query slow operations longer than 200 ms and sort by duration descending.

MongoDB

db.system.profile.find({ [1]: { $gt: 200 } }).sort({ [2]: -1 })

Drag options to blanks, or click blank then click option'

AdurationMillis

Bmillis

DexecutionTime

Attempts:

3 left

💡 Hint

Common Mistakes

Using different field names for filter and sort.

Sorting ascending instead of descending.

Use 'millis' to filter and sort by query duration in milliseconds.

5fill in blank

hard

Fill all three blanks to enable profiling for slow queries over 150 ms and retrieve them sorted by duration.

MongoDB

db.setProfilingLevel([1], { slowms: [2] }); db.system.profile.find({ [3]: { $gt: 150 } }).sort({ [3]: -1 })

Drag options to blanks, or click blank then click option'

A1

B150

Cmillis

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using profiling level 2 which profiles all queries.

Using inconsistent field names for filtering and sorting.

Profiling level 1 enables slow query profiling with slowms 150 ms. Use 'millis' to filter and sort.

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