I have a patch in progress to add SQL:2011 valid time support to Postgres, so that you can more easily record a history of things that change over time. It lets you define temporal primary and foreign keys and issue temporal UPDATE and DELETE commands. Although the SQL:2011 standard introduces a new concept called PERIODs, I've based the Postgres features on our own built-in ranges---and plan to support PERIODs too (hopefully before I get to PGCon).