Joe Celko's SQL for Smarties, 3rd Edition

1.2. Generating Unique Sequential Numbers for Keys

One common vendor extension is using some method of generating a sequence of integers to use as primary keys. These are very nonrelational extensions that are highly proprietary, and have major disadvantages. They all are based on exposing part of the physical state of the machine during the insertion process, in violation of Dr. E. F. Codd’s rules for defining a relational database (i.e., rule 8, physical data independence). Dr. Codd’s rules are discussed in Chapter 2.

Early SQL products were built on existing file systems. The data was kept in physically contiguous disk pages, in physically contiguous rows, made up of physically contiguous columns, in short, just like a deck of punch cards ...