MySQL: Performance issues with complex subquery in WHERE clause using EXISTS
I'm sure I'm missing something obvious here, but I've looked through the documentation and I'm still confused about I'm experiencing significant performance issues when executing a query that uses a subquery with an EXISTS clause in the WHERE condition... The query is intended to filter records based on a related table, but it seems to be taking much longer than expected, especially when working with larger datasets. Here's the simplified version of my query: ```sql SELECT a.id, a.name FROM users a WHERE EXISTS ( SELECT 1 FROM orders b WHERE b.user_id = a.id AND b.status = 'completed' ) AND a.registration_date > '2022-01-01'; ``` This query works fine on smaller datasets, but when I run it on a table with over a million records, the execution time spikes significantly. I've tried adding indexes on both `users.id` and `orders.user_id`, but that didn't seem to improve the performance. Additionally, I’ve tried rewriting the query using a JOIN instead of EXISTS: ```sql SELECT a.id, a.name FROM users a JOIN orders b ON b.user_id = a.id WHERE b.status = 'completed' AND a.registration_date > '2022-01-01'; ``` However, this approach also resulted in a slow performance due to the large number of rows being processed from the orders table. I’m currently using MySQL 8.0.25, and I’ve analyzed the performance with the EXPLAIN command. The output indicates a high number of rows being scanned, which seems to be the root of the issue. Is there a more efficient way to structure my query to avoid this performance hit, or any other strategies I should consider for optimizing queries with complex conditions like this? This is part of a larger web app I'm building. What am I doing wrong? My development environment is Windows.