Comment by default-kramer

(The author is assuming that the primary key controls disk layout, which is usually true.) One advantage of using an incrementing integer is that rows will be ordered on disk based on when they were created. This often helps performance. If a query asks for 25 consecutive rows, there is a good chance they will all be on the same page. If you use UUIDs, then they could be on 25 different pages and you will have to do 25x the disk IO to handle the query.