title: How to extract hour from query in postgres
body: <p>I have timestamp in my table and i want to extract only hour from it. I search and find a extract function but unable to use as a query. Do i need to convert first timestamp in varchar and then extract hour from it? 
  Here is my query:</p>
  
  <pre><code>select extract(hour from timestamp '2001-02-16 20:38:40') // example
  </code></pre>
  
  <p>actual query:</p>
  
  <pre><code>select extract(hour from timestamp observationtime) from smartvakt_device_report
  </code></pre>