是否有“max”函数和分组的替代（聚合操作的性能优化）？

我有一个很大的查询，它也返回很大的响应。查询如下所示：

SELECT group, subgroup, max(last_update) FROM
(
    SELECT a as group, a1 as subgroup, d1 as last_update FROM....
    UNION ALL
    SELECT b as group, b1 as subgroup, d2 as last_update FROM....
    UNION ALL
    SELECT c as group, c1 as subgroup, d3 as last_update FROM....
    UNION ALL
    SELECT d as group, d1 as subgroup, d3 as last_update FROM....
    UNION ALL
    SELECT e as group, e1 as subgroup, d4 as last_update FROM....
    ... and some more selects (15 select queries in total)
) GROUP BY group, subgroup;

如您所见，我需要从某些组加载最大日期。问题是这些日期需要从 15 个选择中加载，而且运行速度非常慢（~4 秒）。我测试了那个子选择

SELECT a as group, a1 as subgroup, d1 as last_update FROM....
UNION ALL
SELECT b as group, b1 as subgroup, d2 as last_update FROM....
UNION ALL
SELECT c as group, c1 as subgroup, d3 as last_update FROM....
UNION ALL
SELECT d as group, d1 as subgroup, d3 as last_update FROM....
UNION ALL
SELECT e as group, e1 as subgroup, d4 as last_update FROM....
... ans some more selects

工作得相当快（~0.1s），问题在于分组功能（这就是查询工作缓慢的原因）：

SELECT group, subgroup, max(last_update) FROM
(
    ...
) GROUP BY group, subgroup;

有什么办法可以改进这个分组吗？正如我所写，目标是获取组中每个子组的最大日期。

也许做group by在每个单独的子查询中？

select g, s, max(last_update) from (
  select g, s, max(last_update) as last_update from t1 group by g, s
  union all
  select g, s, max(last_update) as last_update from t2 group by g, s
  union all
  ...
)
group by g, s

我不能肯定地说，但如果服务器正在为查询构建临时行集，那么这可能会减少该临时行的大小。