title: Postgres No Primary Key Drawback
body: <p>Is there any drawback to not having a primary key for a table in Postgres?  Since all data is stored unordered in the heap anyway, is the primary key just a way to enforce a unique key and an index at the same time?  Or is there a fundamental feature that a primary key provides in a table as opposed to a table that does not have a primary key? </p>