PostgreSQL 14 - Unexpected Behavior with CTEs in Recursive Queries and Performance Degradation
I just started working with I'm relatively new to this, so bear with me... I'm facing a puzzling issue with a recursive Common Table Expression (CTE) in PostgreSQL 14 that seems to be causing significant performance degradation. My query is designed to traverse a hierarchical structure stored in a self-referential table. The intent is to gather all descendants of a particular node, but it takes an unexpectedly long time to execute, especially when the hierarchy depth increases. Here’s the CTE I’m using: ```sql WITH RECURSIVE org_chart AS ( SELECT id, name, manager_id FROM employees WHERE id = $1 -- Starting point, where $1 is the employee ID UNION ALL SELECT e.id, e.name, e.manager_id FROM employees e INNER JOIN org_chart oc ON oc.id = e.manager_id ) SELECT * FROM org_chart; ``` I’ve executed this query with an employee ID of `42`, which should return all subordinates of that employee. However, I noticed that the execution time can reach upwards of 30 seconds for deeper hierarchies, and during this time, I see that the `pg_stat_activity` shows multiple connections waiting on locks or in idle state. To troubleshoot, I’ve checked the execution plan using `EXPLAIN ANALYZE`, but it doesn’t seem to highlight any obvious bottlenecks. The query planner is choosing a sequential scan despite having an index on the `manager_id` column: ```sql CREATE INDEX idx_manager_id ON employees(manager_id); ``` I also tried adjusting the work_mem setting to see if it would help with the performance, but that didn’t yield any improvements. Interestingly, during my tests, I receive intermittent warnings about potential deadlocks, which adds to my confusion. I suspect there might be something off with how the recursive CTE is structured or how the locks are managed in this context. Any insights or suggestions on how to optimize this query for better performance would be greatly appreciated! I'd really appreciate any guidance on this. Any feedback is welcome!