title: extract week number from date postgres
body: <p>I would like to extract the week number as:</p>
  
  <pre><code>2015-52
  </code></pre>
  
  <p>from a date formatted as:</p>
  
  <pre><code>2015-12-27
  </code></pre>
  
  <p>How can I perform this in postgres?</p>
  
  <p>my weeks are calculated from monday to sunday.</p>