title: Within a trigger function, how to get which fields are being updated
body: <p>Is this possible? I'm interested in finding out which columns were specified in the <code>UPDATE</code> request regardless of the fact that the new value that is being sent may or may not be what is stored in the database already.</p>
  
  <p>The reason I want to do this is because we have a table that can receive updates from multiple sources. Previously, we weren't recording which source the update originated from. Now the table stores which source has performed the most recent update.  We can change some of the sources to send an identifier, but that isn't an option for everything. So I'd like to be able to recognize when an <code>UPDATE</code> request doesn't have an identifier so I can substitute in a default value.</p>