Identifying expensive queries that are prime candidates for optimization is a critical step in MySQL performance troubleshooting. These are typically queries that consume a disproportionate amount of resources or take a long time to execute. Here's a structured approach to identifying such queries:

1. Use the Slow Query Log

• Enable Slow Query Log: The slow query log is an invaluable tool for capturing queries that exceed a specified execution time threshold.
• Configuration: Set the long_query_time parameter to define what constitutes a "slow" query. A common starting point is queries taking longer than 1 second, but this can be adjusted based on your environment.
• Log Analysis: Use tools like pt-query-digest to analyze the slow query log and identify the most resource-intensive queries.

2. Monitor Server Performance Metrics

• CPU and Memory Usage: High CPU or memory usage can often point to inefficient queries. Tools like MySQL Workbench or Performance Schema can help identify these resource spikes.
• I/O Utilization: Excessive disk reads or writes can also indicate inefficient queries, particularly those missing proper indexes.

3. Query Execution Plans with EXPLAIN

• EXPLAIN Command: Use EXPLAIN or EXPLAIN ANALYZE for queries to view their execution plans. Look for queries that do full table scans, have inefficient join operations, or appear to use indexes ineffectively.

4. Review Application Logs

• Application-Level Insights: Often, application logs can provide context for slow performance and help identify problematic queries, especially in web applications or services where database interaction is a performance-critical aspect.

5. Performance Schema and sys Schema

• Performance Schema: This feature provides detailed insights into query execution and resource usage.
• sys Schema: The sys schema simplifies querying Performance Schema data and can quickly highlight problematic queries.

6. Regular Health Checks

• Scheduled Audits: Conduct regular performance audits of your database to preemptively identify and optimize expensive queries.
• Change Impact Analysis: After any significant schema change or application update, analyze query performance to catch any new inefficiencies.

7. Client-Side Analysis

• Application Profiling: Profile the database interactions in your application. Often, the issue might not be a single query but a pattern of queries or transactions.

8. Using Third-Party Monitoring Tools

• Comprehensive Monitoring Solutions: Tools like Percona Monitoring and Management (PMM), SolarWinds Database Performance Analyzer, or New Relic offer comprehensive insights and can help in pinpointing expensive queries.

Conclusion

Identifying expensive queries in MySQL involves a combination of log analysis, performance monitoring, execution plan analysis, and regular database health checks. This multi-faceted approach ensures that you not only identify the most resource-intensive queries but also understand the context behind their performance, leading to more effective optimization strategies. Remember, performance optimization is an ongoing process and should be an integral part of database maintenance routines.