Optimize row validation to avoid SELECT * in each level

[nehanene15]

Currently a row validation generates a query like so:

SELECT *, TO_HEX(SHA256(`concat__all`)) AS `hash__all`
FROM (
  SELECT *,
         ARRAY_TO_STRING([`upper__rstrip__ifnull__cast__id`, `upper__rstrip__ifnull__cast__ts`, `upper__rstrip__ifnull__cast__order_date`], '') AS `concat__all`
  FROM (
    SELECT *,
           upper(`rstrip__ifnull__cast__id`) AS `upper__rstrip__ifnull__cast__id`,
           upper(`rstrip__ifnull__cast__ts`) AS `upper__rstrip__ifnull__cast__ts`,
           upper(`rstrip__ifnull__cast__order_date`) AS `upper__rstrip__ifnull__cast__order_date`
    FROM (
      SELECT *, rtrim(`ifnull__cast__id`) AS `rstrip__ifnull__cast__id`,
             rtrim(`ifnull__cast__ts`) AS `rstrip__ifnull__cast__ts`,
             rtrim(`ifnull__cast__order_date`) AS `rstrip__ifnull__cast__order_date`
      FROM (
        SELECT *,
               IFNULL(`cast__id`, 'DEFAULT_REPLACEMENT_STRING') AS `ifnull__cast__id`,
               IFNULL(`cast__ts`, 'DEFAULT_REPLACEMENT_STRING') AS `ifnull__cast__ts`,
               IFNULL(`cast__order_date`, 'DEFAULT_REPLACEMENT_STRING') AS `ifnull__cast__order_date`
        FROM (
          SELECT *, CAST(`id` AS STRING) AS `cast__id`,
                 CAST(`ts` AS STRING) AS `cast__ts`,
                 CAST(`order_date` AS STRING) AS `cast__order_date`
          FROM `pso-kokoro-resources.hivetest.test_timestamp`
        ) t4
      ) t3
    ) t2
  ) t1
) t0

We can optimize the results returned by the query if we get rid of the SELECT * at each level.