It's not necessarily about locking, it's a way to control what order joins get done in, which, at least in MS SQL, is up to the query engine to decide. By writi

It's not necessarily about locking, it's a way to control what order joins get done in, which, at least in MS SQL, is up to the query engine to decide. By writing to a temp table you can override the query engine's plan, often with huge performance gains. Less known is that you can get the same effect using a TOP N statement where N is a number larger than the rows you might get back. This presumably avoids the additional i/o penalty of the temp table creation, but I've never validated that. This trick likely is portable across db engines, too. See https://bertwagner.com/posts/does-the-join-order-of-my-tables-matter/ https://bertwagner.com/posts/does-the-join-order-of-my-table... - Adam Machanic has some good videos diving deeper into this.