Unexpected results when querying with JOINs across large tables in PostgreSQL 13.3
I'm experiencing unexpected results when performing a JOIN operation between two large tables in PostgreSQL 13.3... I have a `users` table with around 1 million rows and an `orders` table with over 5 million rows. The `users` table has a foreign key reference in the `orders` table, and I'm trying to join them to get a list of users along with their order counts. Here's the query I'm using: ```sql SELECT u.id, u.name, COUNT(o.id) AS order_count FROM users u LEFT JOIN orders o ON u.id = o.user_id GROUP BY u.id, u.name ORDER BY order_count DESC; ``` However, I'm getting results that seem to duplicate user entries when there are multiple orders for those users. For instance, if a user has placed three orders, they appear three times instead of once with an order count of three. I've also tried using `DISTINCT` like this: ```sql SELECT DISTINCT u.id, u.name, COUNT(o.id) AS order_count FROM users u LEFT JOIN orders o ON u.id = o.user_id GROUP BY u.id, u.name ORDER BY order_count DESC; ``` But that doesn't resolve the scenario. I also verified that there are no duplicate rows in the `users` table. The scenario continues, and the results seem to vary each time I run the query. I have analyzed the execution plan using `EXPLAIN ANALYZE`, and it shows that the join is running sequentially, which makes me suspect that the indexing might be a question. I currently have the following indexes: ```sql CREATE INDEX idx_users_id ON users(id); CREATE INDEX idx_orders_user_id ON orders(user_id); ``` The indexes are in place, but the performance is still suboptimal and the results are not as expected. Can someone guide to understand what might be going wrong here? Is there a better way to structure this query or optimize it for performance while getting the correct results? I'm working on a service that needs to handle this. Any help would be greatly appreciated! I'm developing on Windows 10 with Sql.