title: Postgres: select the sum of values and then sum this again
body: <p>I tried a lot but can´t find the right way.
  If I select values in Postgres and sum them it looks like this:</p>
  
  <pre><code>SELECT name,sum(size) as total
  FROM mytable group by name order by name;
  </code></pre>
  
  <p>How can I alter this so it also sum all values in total? I think I need a subselect but how?</p>