Oracle PLSQL Datestamp 相减得到毫秒差

Oracle DateStamp 类型时间相减，得到毫秒结果小例子：

-- Created on 11/6/2013 by ZHENG.LIU 
DECLARE
  -- Local variables here
  i       INTEGER := 1;
  t_start TIMESTAMP;
  t_end   TIMESTAMP;

  d_start DATE;
  d_end   DATE;

BEGIN
  -- Test statements here
  SELECT systimestamp INTO t_start FROM dual;
  SELECT SYSDATE INTO d_start FROM dual;

  -- WHILE (i < 100000) LOOP
  --UPDATE dds_fms_config SET VALUE = 'N' WHERE NAME = 'XML_VALIDATION';
  -- i := i + 1;
  --END LOOP;
  SELECT systimestamp INTO t_end FROM dual;
  SELECT SYSDATE INTO d_end FROM dual;

  --Print:
  dbms_output.put_line('[Timestamp]t_start = ' || t_start);
  dbms_output.put_line('[Timestamp]t_end = ' || t_end);

  dbms_output.put_line('[Timestamp - String]t_start = ' ||
                       to_char(t_start, 'yyyy-mm-dd hh24:mi:ss.ff'));
  dbms_output.put_line('[Timestamp - String]t_end = ' ||
                       to_char(t_end, 'yyyy-mm-dd hh24:mi:ss.ff'));
  dbms_output.put_line('[Timestamp]Duration  = ' ||
                       (((to_date(substr(to_char(t_end,
                                                 'yyyy-mm-dd hh24:mi:ss.ff'),
                                         1,
                                         19),
                                  'yyyy-mm-dd hh24:mi:ss') -
                       to_date(substr(to_char(t_start,
                                                 'yyyy-mm-dd hh24:mi:ss.ff'),
                                         1,
                                         19),
                                  'yyyy-mm-dd hh24:mi:ss')) * 24 * 3600 * 1000 * 1000 +
                       to_number(substr(to_char(t_end,
                                                  'yyyy-mm-dd hh24:mi:ss.ff'),
                                          21,
                                          6)) -
                       to_number(substr(to_char(t_start,
                                                  'yyyy-mm-dd hh24:mi:ss.ff'),
                                          21,
                                          6)))) / 1000 || ' ms');

  dbms_output.put_line('[Date]d_start = ' || d_start);
  dbms_output.put_line('[Date]d_end = ' || d_end);

  dbms_output.put_line('[Date - String]d_start = ' ||
                       to_char(d_start, 'yyyy-mm-dd hh24:mi:ss'));
  dbms_output.put_line('[Date - String]d_end = ' ||
                       to_char(d_end, 'yyyy-mm-dd hh24:mi:ss'));

  dbms_output.put_line('[Date]Duration = ' ||
                       (d_end - d_start) * 24 * 60 * 60 || ' s');

END;

可以将此方法封装为function, 方便计算调用。。。