Inconsistent handling of COALESCE during validation

SqlCoalesceFunction has a rewriteCall() method which will replace the call with an SqlCase call during the unconditional rewrites stage.

But expanding natural joins and USING conditions is done using COALESCE calls in a later validation stage.

As a result, the following query:

WITH A AS (SELECT 1 AS X)
  SELECT
    COALESCE(A0.X, A1.X),
    *
  FROM A AS A0 NATURAL JOIN A AS A1

is transformed by the validation step into:

WITH `A` AS (SELECT 1 AS `X`)
  SELECT
    CASE WHEN `A0`.`X` IS NOT NULL THEN `A0`.`X` ELSE `A1`.`X` END, 
    COALESCE(`A0`.`X`, `A1`.`X` AS `X0`) AS `X`
  FROM `A` AS `A0` NATURAL INNER JOIN `A` AS `A1`

Maybe, the expanded results of * with natural joins and of USING should pass through unconditional rewrites at creation.