title: What is the best architecture for tracking field changes on objects?
body: <p>We have a web application that is built on top of a SQL database. Several different types of objects can have comments added to them, and some of these objects need field-level tracking, similar to how field changes are tracked on most issue-tracking systems (such as status, assignment, priority).  We'd like to show who the change is by, what the previous value was, and what the new value is.</p>
  
  <p>At a pure design level, it would be most straightforward to track each change from any object in a generic table, with columns for the object type, object primary key, primary key of the user that made the change, the field name, and the old and new values. In our case, these would also optionally have a comment ID if the user entered a comment when making the changes.</p>
  
  <p>However, with how quickly this data can grow, is this the best architecture? What are some methods commonly employed to add this type of functionality to an already large-scale application?</p>
  
  <p>[<strong>Edit</strong>]  I'm starting a bounty on this question mainly because I'd like to find out in particular what is the best architecture in terms of handling scale very well. Tom H.'s answer is informative, but the recommended solution seems to be fairly size-inefficient (a new row for every new state of an object, even if many columns did not change) and not possible given the requirement that we must be able to track changes to user-created fields as well.  In particular, I'm likely to accept an answer that can explain how a common issue-tracking system (JIRA or similar) has implemented this.</p>