title: Postgresql add existing column to composite primary key
body: <p>I have a table in postgresql with a composite primary key. The primary key consists of two columns named:</p>
  
  <pre><code>DATETIME, UID
  </code></pre>
  
  <p>I have a another (non-null) column named ACTION already existing in this table. How do I add ACTION to the composite primary key? Ie: I'd like the resulting primary key of the table to be the triplet:</p>
  
  <pre><code>DATETIME, UID, ACTION
  </code></pre>