以比此查询更快的方式计算数量

Question

我需要从数据库中计算总可用数量,为此我需要与几个表进行连接.我无法粘贴我的整个查询,但以下是基本结构:

select sum(qty) as qty, field
from
(
  (
      select SUM(table1.qty) as qty , field
       from
       table1
       left join table2 on table1.field = table2.field
       left join table3 on table3.field = table2.field
       where condition
       group by fieldname
   )
   UNION ALL
   (
       selecy SUM(table1.qty) as qty,field 
       from
       table1
       left join table2 on table1.field = table2.field
       left join table3 on table3.field = table2.field
       where condition
       group by fieldname
   )
   UNION ALL
   (
       select SUM(table1.qty) as qty, field
       from
       table1
       left join table2 on table1.field = table2.field
       left join table3 on table3.field = table2.field
       where condition
       group by fieldname
   )
  ...
  ..
  12 times
) as temp
LEFT JOIN another_main_table ON another_main_table.field = temp.field

我已经处理了每个表的索引,但是有一些联盟需要比预期更长的时间.此查询中使用了大约45个表,并且所有表都已完全检查.一些表有大约260万条记录.你能告诉我如何在1/2秒内得到结果吗？截至目前,我得到的结果大约一分钟.