Troubleshooting performance issues with ad-hoc queries in MySQL involves identifying and addressing the causes of inefficiencies or delays in query execution. Ad-hoc queries, which are typically one-off or dynamically generated queries, can sometimes perform poorly due to various factors. Here's a guide on how to approach this:

1. Identify Slow Queries

• Enable Slow Query Log: Configure the MySQL server to log slow queries. This is done by setting the slow_query_log variable and defining what constitutes a slow query with the long_query_time setting.
• Review the Slow Query Log: Regularly check the slow query log to identify queries that are taking longer than expected.

2. Analyze Execution Plans

• Use EXPLAIN: Run EXPLAIN on the slow queries to get insights into the query execution plan. This will show how MySQL intends to execute the query, including information about joins, indexes used, and estimated rows examined.
• Analyze Index Usage: Look for queries that are not using indexes efficiently. Queries that result in full table scans can be particularly slow.

3. Optimize Query Structure

• Simplify Complex Queries: Break down complex queries into simpler ones, if possible. Complex joins, subqueries, and nested queries can often be optimized.
• Avoid Selecting Unnecessary Columns: Use SELECT statements to fetch only the columns that are needed, rather than using SELECT *.

4. Optimize Database Schema

• Indexing: Ensure that appropriate indexes are in place for the columns used in WHERE, JOIN, ORDER BY, and GROUP BY clauses.
• Schema Design: Revisit your database schema design. Poorly designed schemas can lead to inefficient queries.

5. Server and Hardware Optimization

• Memory Allocation: Adjust key buffer sizes and memory allocation, like innodb_buffer_pool_size, especially if your database is large.
• Hardware Performance: Consider hardware limitations. Slow disk I/O, insufficient memory, or CPU bottlenecks can impact query performance.

6. Parameter Tuning

• Tweak MySQL Parameters: Adjust MySQL configuration parameters that can impact performance, such as query_cache_size (though note that query cache is deprecated in MySQL 8.0), join_buffer_size, and tmp_table_size.

7. Consider Caching Mechanisms

• Application-Level Caching: Implement caching at the application level for frequently accessed data, especially for read-heavy ad-hoc queries.

8. Regular Maintenance

• Update Statistics: Ensure that statistics used by the query optimizer are up-to-date.
• Database Maintenance: Perform regular maintenance tasks like defragmenting tables and optimizing indexes.

9. Use Monitoring Tools

• Monitoring and Profiling Tools: Use tools like MySQL Workbench, Performance Schema, or third-party monitoring solutions to continuously monitor database performance.

Conclusion

Troubleshooting ad-hoc query performance in MySQL requires a comprehensive approach that includes analyzing query execution plans, optimizing query structures and database schemas, ensuring efficient use of indexes, and possibly adjusting server configuration. Regular monitoring and maintenance are key to identifying and resolving performance issues. In some cases, hardware upgrades or changes to the application architecture may also be necessary to achieve optimal performance.