这应该只在整个表中进行一次扫描：

WITH result AS (
  SELECT
    count(id) AS count_id,
    count(firstname) AS count_firstname,
    count(lastname) AS count_lastname
  FROM person
)
SELECT
  'firstname' AS attr_name,
  count_id,
  count_id - count_firstname AS count_missing,
  count_firstname AS count_total
FROM result
UNION ALL
SELECT
  'lastname' AS attr_name,
  count_id,
  count_id - count_lastname AS count_missing,
  count_lastname AS count_total
FROM result