title: serial in postgres is being increased even though I added on conflict do nothing
body: <p>I'm using Postgres 9.5 and seeing some wired things here.</p>
  
  <p>I've a cron job running ever 5 mins firing a sql statement that is adding a list of records if not existing.</p>
  
  <pre><code>INSERT INTO 
     sometable (customer, balance)
  VALUES
      (:customer, :balance)
  ON CONFLICT (customer) DO NOTHING
  </code></pre>
  
  <p>sometable.customer is a primary key (text)</p>
  
  <p><strong><em>sometable structure is:</em></strong><br>
  id: serial<br>
  customer: text<br>
  balance: bigint   </p>
  
  <p>Now it seems like everytime this job runs, the id field is silently incremented +1. So next time, I really add a field, it is thousands of numbers above my last value. I thought this query checks for conflicts and if so, do nothing but currently it seems like it tries to insert the record, increased the id and then stops.</p>
  
  <p>Any suggestions?</p>