hmmm... offhand, I would guess that your query would work. it doesn't?
You can simplify it like this:
from
ec_product_recommendations recs
LEFT JOIN
ec_categories cats using (category_id)
LEFT JOIN
ec_subcategories_augmented subs using (subcategory_id)
LEFT JOIN
ec_subsubcategories_augmented subsubs
on (recs.subsubcategory_id = subsubs.subcategory_id);
PG nests joins left to right, so it will do the recs-cats join first,
take the results of that and join with subs, etc...