GBase 8s数据库中，两个日期时间的值相减，得到的是interval类型的值（interval day to second），而oracle中直接就是数值了。我们可以通过自定义函数的方式，把interval类型转换成数值型（decimal(20,8)）。

drop function if exists intval2number(interval day to second);
create function intval2number(v_intval interval day to second) returns decimal(20,8)
  define v_numsec bigint;
  define v_decday decimal(20,8);
  define v_intlen int;
  define v_tmpsec int;
  define v_tmpmin int;
  define v_tmphour int;
  define v_tmpday int;
  on exception
    return null;
  end exception;
  -- 对于interval day to second，将其转换为字符型，才能取长度。
  let v_intlen = length(v_intval::varchar(40));
  let v_tmpsec = substr(v_intval, v_intlen - 1, 2);
  let v_tmpmin = substr(v_intval, v_intlen - 4, 2);
  let v_tmphour = substr(v_intval, v_intlen - 7, 2);
  let v_tmpday  = substr(v_intval, 1, v_intlen - 9);
  let v_numsec = v_tmpsec + v_tmpmin * 60 + v_tmphour * 3600 + abs(v_tmpday)::bigint * 86400;
  let v_decday = v_numsec / 86400;
  -- 对于负值的处理
  if v_tmpday < 0 then
    return 0 - v_decday;
  else
    return v_decday;
  end if;  
end function;

相应的，如果输入的是字符串类型，可以写成如下：

drop function if exists intval2number(varchar(40));
create function intval2number(p varchar(40)) returns decimal(20,8)
  define v_intval varchar(40);
  define v_numsec bigint;
  define v_decday decimal(20,8);
  define v_intlen int;
  define v_tmpsec int;
  define v_tmpmin int;
  define v_tmphour int;
  define v_tmpday int;
  on exception
    return null;
  end exception;
  let v_intval = to_dsinterval(p);
  let v_intlen = length(v_intval);
  let v_tmpsec = substr(v_intval, v_intlen - 1, 2);
  let v_tmpmin = substr(v_intval, v_intlen - 4, 2);
  let v_tmphour = substr(v_intval, v_intlen - 7, 2);
  let v_tmpday  = substr(v_intval, 1, v_intlen - 9);
  let v_numsec = v_tmpsec + v_tmpmin * 60 + v_tmphour * 3600 + abs(v_tmpday)::bigint * 86400;
  let v_decday = v_numsec / 86400;
  -- 对于负值的处理
  if v_tmpday < 0 then
    return 0 - v_decday;
  else
    return v_decday;
  end if;  
end function;

示例：
输入参数为字符串

select intval2number('-111111111 00:00:01') from dual;

的结果为：

(expression)        |
--------------------|
-111111111.00001157 |

输入参数为interval类型时

select intval2number(current year to second - today) from dual;

的结果为：

(expression) |
-------------|
0.54226852   |