title: How to Group By using Month from date stored as millisecond Postgres
body: <p>I have a transaction table which contains <code>net_amount</code> field and <code>last_updated_time</code> field, where <code>last_updated_time</code> is stored as milliseconds. I need to get total amount group by using month, year or date. How can I do this in PostgreSQL?</p>
  
  <p>My table looks like as below:</p>
  
  <pre><code>+------------+-------------------+
  | net_amount | last_updated_time |
  +------------+-------------------+
  | 100        | 1470286872831     |
  +------------+-------------------+
  | 200        | 1471594713801     |
  +------------+-------------------+
  | 300        | 1471594651335     |
  +------------+-------------------+
  </code></pre>
  
  <p>and expecting result as:</p>
  
  <pre><code>+----------+---------------+
  | month    | sum_of_amount |
  +----------+---------------+
  | january  | 1000          |
  +----------+---------------+
  | february | 2000          |
  +----------+---------------+
  | ---      | ----          |
  +----------+---------------+
  </code></pre>