A scan node sits at the leaf of the tree and pulls rows from a single table. A join node sits in the middle and brings together the rows that its two children send up. It takes one row from users , one row from orders , checks whether they belong to the same user, and if they match, emits the combined row. PostgreSQL has three nodes for this one job: NestLoop, HashJoin, and MergeJoin. The reason a single task splits into three nodes is much like the reason scans did. There is more than one way t