mySQL Quickly find rows linked to another row
Presumably, any given user can only like a given post once, so the likes table does not need and should not have an id column... Instead, define that table with PRIMARY KEY (post_id,user_id).  The likes for a given post will be quickly retrievable using the leftmost column of the primary key for lookups and you then have automatic prevention of duplicate likes for the same post by the same user as a secondary benefit.