title: Postgres timestamp to unix time in milliseconds as a bigint
body: <p>How can I get the following snippet to work in postgres:</p>
  
  <pre><code>ALTER TABLE mytable
  ADD COLUMN create_time_utc bigint not null
  DEFAULT (now() at time zone 'utc');
  </code></pre>
  
  <p>I want the new column <code>create_time_utc</code> to be the unix time in milliseconds (i.e number of milliseconds since Unix epoch January 1 1970).</p>
  
  <p>I know I need to convert the postgres <code>timestamp</code> to a bigint, but I'm not sure how to do that.</p>