title: Postgres could not serialize access due to concurrent update
body: <p>I have an issue with "<em>could not serialize access due to concurrent update</em>". I checked logs and I can clearly see that two transactions were trying to update a row at the same time. </p>
  
  <p>my sql query</p>
  
  <pre><code>UPDATE sessionstore SET valid_until = %s WHERE sid = %s;
  </code></pre>
  
  <p>How can I tell postgres to "try" update row without throwing any exception? </p>