# n is normally an integer but can be anything
create or replace function sum_digits(n) as (
  regexp_extract_all(n::VARCHAR, '.')
    .list_transform( x -> (ascii(x) - 48) % 10)
    .list_sum()
);

# n should be an integer
create or replace function digital_root(n) as table (
  with recursive cte as (
    select 0 as ix, n as sum
    union all
    select ix+1, sum_digits(sum)
    from cte
    where length(sum::VARCHAR) > 1
  )
  select last(ix order by ix) as persistence,
         last(sum order by ix) as "digital root",
  from cte
);

## Examples:
from (select unnest(range(0, 4)) as i,
      unnest([627615, 39390, 588225, 393900588225]) as n),
     (from digital_root(n))
order by i;