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 c...
Source: [Dev.to](https://dev.to/joonghyukshin/153-join-nodes-nestloop-hashjoin-mergejoin-2fek)