MySQL 5.7 - Performance Degradation with JOIN on Large Tables and IN Clause
I've been banging my head against this for hours... I'm writing unit tests and Could someone explain I've been banging my head against this for hours. I'm experiencing important performance optimization when executing a query that joins two large tables in MySQL 5.7. The query looks something like this: ```sql SELECT a.id, a.name, b.order_date FROM users a JOIN orders b ON a.id = b.user_id WHERE a.status = 'active' AND b.product_id IN (SELECT id FROM products WHERE category = 'books'); ``` The `users` table has about 1 million records, and the `orders` table has around 10 million records. I have indexes on `user_id` in the `orders` table and `status` in the `users` table. However, the execution time is around 30 seconds, and it spikes the CPU usage. I tried using `EXPLAIN` on the query, and it shows a `Using temporary` and `Using filesort`, which I suspect is causing the slowdown. To troubleshoot, I also tried running the inner subquery separately, which executes quickly, but the overall query is still inefficient. I even attempted to rewrite the query using a `JOIN` instead of the `IN` clause: ```sql SELECT a.id, a.name, b.order_date FROM users a JOIN orders b ON a.id = b.user_id JOIN products p ON b.product_id = p.id WHERE a.status = 'active' AND p.category = 'books'; ``` This also did not yield much improvement, and I still observed the same performance optimization. Are there any best practices or tuning techniques I might be missing to optimize such JOIN queries in MySQL 5.7? Any insight would be greatly appreciated! This is part of a larger CLI tool I'm building. Is there a better approach? I'm working on a microservice that needs to handle this. I'd really appreciate any guidance on this. This issue appeared after updating to Sql stable. I'm coming from a different tech stack and learning Sql. Any suggestions would be helpful.