MySQL 8.0 - Performance Degradation with CTEs on Large Data Sets
I'm stuck on something that should probably be simple. I'm converting an old project and I'm experiencing important performance degradation when using Common Table Expressions (CTEs) in MySQL 8.0 with large datasets. I have a query that uses a recursive CTE to retrieve hierarchical data, and it was performing adequately with smaller data sets. However, when I scale up to around 500,000 rows, the query execution time increases dramatically, taking over 30 seconds, while a similar query without CTEs runs in just a couple of seconds. Here’s the CTE I’m working with: ```sql WITH RECURSIVE category_hierarchy AS ( SELECT id, name, parent_id FROM categories WHERE parent_id IS NULL UNION ALL SELECT c.id, c.name, c.parent_id FROM categories c INNER JOIN category_hierarchy ch ON ch.id = c.parent_id ) SELECT * FROM category_hierarchy; ``` I’ve tried adding indexes on `parent_id`, but it didn’t seem to make much difference. I've also experimented with increasing the `max_execution_time` setting, but it only pushes the limits without solving the underlying scenario. I suspect that the recursive nature of the query is causing the performance hit, but I’m not sure how to optimize it. Additionally, I see a warning in the logs stating `Warning | The query is using a temporary table, which may affect performance`. Could this be related to the way the CTE is structured? Any advice on how to optimize this CTE or alternative approaches to achieve the same result would be greatly appreciated. This issue appeared after updating to Sql 3.10. The stack includes Sql and several other technologies.