Monitoring and profiling in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Identify slow queries using MySQL slow query log

You enabled the slow query log in MySQL with the following settings:

slow_query_log = ON
slow_query_log_file = /var/log/mysql/mysql-slow.log
long_query_time = 2

What will be the output of the command below?

tail -n 5 /var/log/mysql/mysql-slow.log

A Shows the first 5 queries executed since MySQL started

B Shows the last 5 queries regardless of execution time

C Shows an error because the slow query log file path is incorrect

D Shows the last 5 slow queries that took longer than 2 seconds to execute

💡 Hint

The slow query log records queries that exceed the long_query_time threshold.

The slow query log file contains queries that took longer than the configured long_query_time (2 seconds here). The tail command shows the last 5 entries from this log.

❓ Configuration

intermediate

Configure MySQL to enable performance schema for profiling

Which configuration snippet correctly enables the Performance Schema in MySQL for detailed query profiling?

A

[mysqld]
performance_schema=TRUE

B

[mysqld]
performance_schema=ON

C

[mysqld]
performance_schema=1

D

[mysqld]
enable_performance_schema=ON

💡 Hint

Check the exact variable name and accepted values in MySQL documentation.

The correct variable to enable Performance Schema is 'performance_schema' and it accepts 'ON' or 'OFF' as values.

❓ Troubleshoot

advanced

Diagnose why slow query log is not recording queries

You enabled the slow query log with:

SET GLOBAL slow_query_log = 'ON';
SET GLOBAL long_query_time = 1;

But no queries appear in the slow query log file. What is the most likely cause?

A The MySQL user does not have write permission to the slow query log file location

B The long_query_time is set too low to capture any queries

C The slow query log requires a server restart to take effect

D The slow query log file path is not set, so logging is disabled

💡 Hint

Check file permissions and ownership for the log file path.

If MySQL cannot write to the slow query log file due to permission issues, no queries will be logged even if logging is enabled.

🔀 Workflow

advanced

Steps to profile a query using Performance Schema

Arrange the steps in the correct order to profile a slow query using MySQL Performance Schema.

A 1,2,3,4

B 2,1,3,4

C 1,3,2,4

D 3,1,2,4

💡 Hint

Profiling requires enabling, running, then analyzing.

First enable Performance Schema and restart MySQL. Then run the query to collect data. Next, query the Performance Schema tables. Finally, analyze the data.

✅ Best Practice

expert

Best practice for continuous MySQL query performance monitoring

Which approach is best for continuous monitoring of MySQL query performance in a production environment?

A Enable slow query log with a low long_query_time and manually analyze logs daily

B Disable all logging to maximize performance and only enable logs during incidents

C Use Performance Schema with automated tools to collect and visualize query metrics continuously

D Run EXPLAIN on all queries manually every week to check performance

💡 Hint

Consider automation and minimal manual effort for continuous monitoring.

Performance Schema combined with automated monitoring tools provides continuous, detailed insights with minimal manual work, ideal for production.

Practice

1. What is the main purpose of SHOW PROCESSLIST in MySQL monitoring?

easy

A. To display currently running queries and their status

B. To list all database users

C. To show the database schema

D. To backup the database

Solution

1. Step 1: Understand the command purpose

   SHOW PROCESSLIST lists all active threads and queries running on the MySQL server.

2. Step 2: Identify what it displays

   It shows query IDs, user, host, command, time, and status of each running query.

3. Final Answer:

   To display currently running queries and their status -> Option A

4. Quick Check:

   SHOW PROCESSLIST = Running queries info [OK]

Hint: SHOW PROCESSLIST shows active queries and their states [OK]

Common Mistakes:

• Confusing it with user listing commands
• Thinking it shows database structure
• Assuming it performs backups

2. Which of the following is the correct syntax to enable the MySQL slow query log?

easy

A. SET GLOBAL slow_query_log = ON;

B. SET GLOBAL slow_query_log = 1;

C. ENABLE slow_query_log;

D. START slow_query_log;

Solution

1. Step 1: Recall the correct syntax for enabling slow query log

   MySQL uses numeric values 0 (off) and 1 (on) for boolean system variables.

2. Step 2: Check each option

   SET GLOBAL slow_query_log = 1; uses SET GLOBAL slow_query_log = 1; which is correct syntax to enable the slow query log.

3. Final Answer:

   SET GLOBAL slow_query_log = 1; -> Option B

4. Quick Check:

   Enable slow query log with 1, not ON [OK]

Hint: Use 1 or 0 to enable/disable slow query log [OK]

Common Mistakes:

• Using ON instead of 1
• Using ENABLE or START commands which don't exist
• Confusing session and global variables

3. Given the query:

SHOW STATUS LIKE 'Slow_queries';

What does the output number represent?

medium

A. The maximum time a query took to execute

B. The total number of queries executed since server start

C. The number of currently running slow queries

D. The number of queries that took longer than the slow query time threshold

Solution

1. Step 1: Understand the status variable 'Slow_queries'

   This variable counts how many queries exceeded the slow query time limit since the server started.

2. Step 2: Interpret the output meaning

   The number is cumulative, showing total slow queries, not current or max time.

3. Final Answer:

   The number of queries that took longer than the slow query time threshold -> Option D

4. Quick Check:

   Slow_queries = count of slow queries [OK]

Hint: Slow_queries counts total slow queries since server start [OK]

Common Mistakes:

• Thinking it shows current running slow queries
• Confusing it with total queries count
• Assuming it shows max query time

4. You run SHOW PROFILE; but get an error saying profiling is disabled. What should you do to fix this?

medium

A. Run SET profiling = 1; before running queries

B. Restart MySQL server to enable profiling

C. Run ENABLE PROFILING; command

D. Profiling is deprecated and cannot be enabled

Solution

1. Step 1: Understand profiling status

   Profiling is off by default and must be enabled per session with SET profiling = 1;.

2. Step 2: Identify correct enabling method

   Run SET profiling = 1; before running queries correctly enables profiling for the current session before running queries.

3. Final Answer:

   Run SET profiling = 1; before running queries -> Option A

4. Quick Check:

   Enable profiling with SET profiling = 1 [OK]

Hint: Enable profiling per session with SET profiling = 1 [OK]

Common Mistakes:

• Trying to enable profiling globally or via restart
• Using ENABLE PROFILING command which doesn't exist
• Assuming profiling is permanently on

5. You want to find the top 3 slowest queries executed on your MySQL server using the slow query log. Which approach is best?

hard

A. Run SHOW PROCESSLIST and sort by longest running queries

B. Use EXPLAIN on all queries to find slow ones

C. Enable slow query log, then query the mysql.slow_log table ordering by query_time DESC LIMIT 3

D. Restart MySQL with debug mode to log slow queries

Solution

1. Step 1: Enable slow query log and log to table

   Set slow_query_log = 1 and log_output = 'TABLE' to log slow queries into mysql.slow_log table.

2. Step 2: Query the slow_log table for top slow queries

   Run SELECT * FROM mysql.slow_log ORDER BY query_time DESC LIMIT 3; to get the slowest queries.

3. Final Answer:

   Enable slow query log, then query the mysql.slow_log table ordering by query_time DESC LIMIT 3 -> Option C

4. Quick Check:

   Slow query log table + ORDER BY query_time DESC = top slow queries [OK]

Hint: Query slow_log table ordered by query_time DESC for slowest queries [OK]

Common Mistakes:

• Using SHOW PROCESSLIST which shows only current queries
• Using EXPLAIN which analyzes query plan, not runtime
• Restarting server unnecessarily for logging