The database itself may internally store a more dense identifier than an autoincrementing primary key. For e.g. ctid in Postgres or, under certain conditions ro

The database itself may internally store a more dense identifier than an autoincrementing primary key. For e.g. ctid in Postgres or, under certain conditions rowid in sqlite. These should be fully dense after a vacuum, and rejection sampling can be used to paper over tombstones generated between vacuums.