inc5.sql

CREATE FUNCTION inc5 (num NUMERIC) 
    returns NUMERIC 
    language SQL
AS $$
    WITH recursive n5 (i, l, nt, ln, c) AS (
        SELECT -1, LENGTH(num::text)+1, num::text, '', 1
        UNION ALL
        SELECT i+1, l - 1, nt, 
            CASE (substring(nt FROM l-1 FOR 1)::INT + c)
                WHEN 5
                THEN '0'
                ELSE (substring(nt FROM l-1 FOR 1)::INT + c)::text
            END, 
            CASE (substring(nt FROM l-1 FOR 1)::INT + c)
                WHEN 5
                THEN 1
                ELSE 0
            END
        FROM n5
        WHERE l > 1
    )
    SELECT string_agg(digit, '')::NUMERIC
    FROM (SELECT ln AS digit FROM n5 WHERE i >= 0 ORDER BY i DESC) ch;
$$;