CREATE OR REPLACE FUNCTION limit_download_func() RETURNS TRIGGER
    AS $$
    BEGIN
        IF ((SELECT COUNT(*) FROM download WHERE user_id = NEW.user_id AND create_at = now()) < 2) THEN
            INSERT INTO "download" ("object_id", "user_id") VALUES (NEW.object_id, NEW.user_id);
            RETURN NEW;
        ELSE
            RAISE EXCEPTION
                'Превышено количество максимальных обращений для юзера %!', NEW.user_id;
        END IF;
    END;
$$ LANGUAGE plpgSQL;


CREATE TRIGGER limit_download
    before INSERT ON download
        FOR STATEMENT EXECUTE PROCEDURE limit_download_func();