Is this a possible attack on database?
I asked a member of the Drupal security team about this and he referred me to https://www.drupal.org/drupalsa05FAQ#comment-9255523.

This points to a user attempting to exploit SA CORE 2014 005, but failing due to the site being patched. Cesar's attempt to reproduce triggers the PDOException, but does not (and cannot) result in actual SQL injection.