SELECT AllPossible.ID
FROM T RIGHT JOIN (
    SELECT @i:=@i+ 1  AS ID
    FROM T, T Self, (SELECT @i:= 0 ) AS Z
) AS AllPossible ON T.ID=AllPossible.ID
WHERE T.ID IS NULL AND AllPossible.ID <= (
    SELECT MAX(T.ID)
    FROM T
);

FROM T, T Self

FROM T CROSS JOIN T Self

WHERE ... AND AllPossible.ID <= (
    SELECT MAX(T.ID)
    FROM T
);

SELECT @i:=@i+ 1  AS ID
FROM T, T Self, (SELECT @i:= 0 ) AS Z

SELECT t1.id
FROM t
  RIGHT JOIN (
    SELECT ti.id
    FROM (
        SELECT @rownum := @rownum +  1  id
        FROM (SELECT @rownum :=  0  rownum) v,
          (SELECT null UNION ALL SELECT null) tii1,
          (SELECT null UNION ALL SELECT null) tii2,
          ...
          (SELECT null UNION ALL SELECT null) tiiM
      ) ti
    WHERE ti.id <= N
  ) t1 ON t.id = t1.id
WHERE t.id IS NULL

USE test;
CREATE TABLE test(
  id INT PRIMARY KEY
)
          SELECT   1  id
UNION ALL SELECT   2 
UNION ALL SELECT   5 
UNION ALL SELECT   6 
UNION ALL SELECT   9 
UNION ALL SELECT  10 ;

SELECT SUBSTR(@result,  3 ) result
FROM (
    SELECT BENCHMARK(t.id, (SELECT LEAST( 0 , @rownum := @rownum +  1 , CASE WHEN NOT EXISTS(
        SELECT null FROM test tii WHERE tii.id = @rownum)
            THEN @result := CONCAT_WS(', ', @result, @rownum) END)))
    FROM (SELECT @rownum :=  0 , @result := '') v, (SELECT MAX(ti.id) id FROM test ti) t
  ) tt;

DROP TABLE test;

SELECT MAX(ti.id) id FROM test ti) t

select its_a_hole_id
from (
    select level its_a_hole_id
    from dual
    connect by level <= (
        select max(tbl.natural_id_with_holes)
        from your_table_with_holes tbl
    )
) gnr
where not exists (
    select natural_id_with_holes
    from your_table_with_holes tbl
    where tbl.natural_id_with_holes = gnr.its_a_hole_id)

SELECT SUBSTR(@result,  3 ) result
FROM (
    SELECT BENCHMARK(t.id, (SELECT LEAST( 0 , @rownum := @rownum +  1 , CASE WHEN NOT EXISTS(
        SELECT null FROM T tii WHERE tii.id = @rownum)
            THEN @result := CONCAT_WS(', ', @result, @rownum) END)))
    FROM (SELECT @rownum :=  0 , @result := '') v, (SELECT MAX(ti.id) id FROM T ti) t
  ) tt;

(SELECT MAX(ti.id) id FROM T ti) t