title: PostgreSQL - Determine which columns were updated
body: <p>I have a table with many columns, one of which is a <code>lastUpdate</code> column.
  I am writing a trigger in plpgsql for Postgres 9.1, that should set a value for <code>lasUpdate</code> upon an UPDATE to the record. </p>
  
  <p>The challenge is to exclude some pre-defined columns from that trigger; Meaning, updating those specific columns shouldn't affect the <code>lastUpdate</code> value of the record.</p>
  
  <p>Any advise?</p>