Doctrine.union.php

<?php 
public function getAllIds()
{
        $expr = $this->_em->getExpressionBuilder();

        $rsm = new ResultSetMapping();
        $rsm->addScalarResult('id', 'id');

        $result = $this->_em->createNativeQuery('
                SELECT u.id as id FROM users u
                  UNION
                SELECT a.id as id FROM social_account a
        ', $rsm)->iterate();

        foreach ($result as $item) {
            $dto = array_shift($item);
            yield new Id($dto['id']);
        }
}