The problem was caused by using improper column names. Symfony entity generator change names for example from id_product to idProduct.
So:
SELECT p, s FROM DemoProductBundle:Product p
LEFT JOIN DemoProductBundle:SynchronizationSetting s WITH p.id = s.id_product ORDER BY p.id ASC
shoud be:
SELECT p, s FROM DemoProductBundle:Product p
LEFT JOIN DemoProductBundle:SynchronizationSetting s WITH p.id = **s.idProduct** ORDER BY p.id ASC
Now works fine.