Troubleshooting performance issues with SELECT * queries in MySQL 8 involves several steps to diagnose and improve query efficiency. SELECT * queries can be problematic because they retrieve all columns from a table, which can lead to unnecessary data processing and network overhead, especially for tables with many columns or large column data types. Here's a step-by-step approach to troubleshoot these queries:

1. Analyze the Query with EXPLAIN

• Use the EXPLAIN statement to get insights into how MySQL executes the query. This will show if the query is using indexes effectively and provide details on joins, sorting, and other operations.

  EXPLAIN SELECT * FROM your_table;
  
  

2. Review Table Schema

• Examine the table schema. Large columns, especially BLOBs or TEXT types, can significantly impact the performance of SELECT * queries.
• Consider normalizing the table if it contains denormalized or redundant data.

3. Limit Columns in SELECT

• Modify the query to select only the necessary columns instead of using ``. This reduces the amount of data transferred and processed.

  SELECT column1, column2, column3 FROM your_table;
  
  

4. Check Indexes

• Ensure that the table has appropriate indexes for the columns used in WHERE, ORDER BY, GROUP BY, and JOIN clauses.
• Avoid indexing every column as it can increase storage and affect write performance.

5. Evaluate Joins

• If your SELECT * query involves joins, make sure they are optimized. Use EXPLAIN to check if indexes are being used for joins.
• Consider the order of tables in joins; in some cases, reordering the tables can improve performance.

6. Pagination for Large Result Sets

• For large result sets, use pagination to limit the number of rows returned in a single query. This can be done using LIMIT and OFFSET.

  SELECT * FROM your_table LIMIT 100 OFFSET 200;
  
  

7. Use Query Caching

• MySQL 8.0 has deprecated the query cache, but for earlier versions or in some forks of MySQL, query caching can improve performance for frequent identical queries.

8. Server and Hardware Configuration

• Ensure your MySQL server configuration is optimized for your workload (innodb_buffer_pool_size, sort_buffer_size, etc.).
• Consider server hardware upgrades if the server is resource-constrained (CPU, memory, storage).

9. Monitor Server Performance

• Use tools like MySQL Workbench, Performance Schema, or SHOW PROCESSLIST to monitor server performance and identify slow queries.

10. Query Refactoring

• In some cases, rewriting the query or splitting it into multiple queries can improve performance.

Conclusion

Troubleshooting SELECT * queries in MySQL 8 mainly revolves around avoiding unnecessary data retrieval, ensuring efficient use of indexes, optimizing joins, and server configuration. Regular monitoring and query analysis are crucial to maintaining optimal performance. Remember, each query should be tailored to retrieve only the data necessary for the specific application need.