使用查询结果设置下一个查询表

Question

我有以下查询，返回两个不同表中基本唯一的行的最小值，最大值和用户总数。

select sum(user_count) as user_count_sum, sum(min_count) as min_count_sum, 
sum(max_count) as max_count_sum
from(
select max(case when di = 'i' then di_count end) as user_count, 
       min(di_count) as min_count,
       max(di_count) as max_count
from (
    select di, 
           count(distinct rt) as di_count
    from gpstablev2 
    where rt >  GETDATE() - '1 day'::INTERVAL
    group by di
)
UNION
select max(case when di = 'i' then di_count end) as user_count, 
       min(di_count) as min_count,
       max(di_count) as max_count
from (
    select di, 
           count(distinct rt) as di_count
    from powertablev2 
    where rt >  GETDATE() - '1 day'::INTERVAL
    group by di
)
)

现在我还有一个主表，如果我要运行以下查询，将返回以下内容

select table from mastertable;

gpstablev2
powertablev2
...(more table names)

不必像粘贴9个以上的UNION块那样，我必须对mastertable列出的所有表mastertable ，有没有一种方法可以使用查询来对mastertable进行清理呢？

如果我在Java中执行此操作，则需要先从主表中获取ResultSet，然后再执行10个查询，然后自己对它们进行UNION 。 但是我宁愿我的数据库进行所有处理，并能够基于mastertable的内容进行mastertable 。

编辑：根据反馈，我正在尝试动态查询，并且已经走到了这一步，但是仍然很难将其余部分组合在一起

CREATE OR REPLACE FUNCTION get_sums()
RETURNS TABLE(user_count_sum bigint, min_count_sum bigint, max_count_sum bigint)
$BODY$
BEGIN

RETURN QUERY EXECUTE "
SELECT $$SELECT sum(user_count) AS user_count_sum
      ,sum(min_count) AS min_count_sum
      ,sum(max_count) AS max_count_sum
FROM  (
   SELECT max(case when di = 'id' then di_count end) AS user_count
         ,min(di_count) AS min_count
         ,max(di_count) AS max_count
   FROM  ($$
||
string_agg(format($$
      (SELECT di, count(distinct rt) AS di_count
       FROM   %I
       WHERE  rt >  now() - interval '1 day'
       GROUP  BY 1)$$, tbl)
      ,'

      UNION ALL')
|| '
      ) sub1
   ) sub2'
FROM   mastertable;
INTO results_var"

END;
$BODY$
LANGUAGE plpqsql;

Answer 1

为此，您需要动态SQL 。 SQL不接受文字/值到标识符的动态转换。 因此，您需要先构建查询然后执行它。 您可以在客户端（似乎是Java）中执行此操作，也可以在Postgres中执行所有操作 ，这通常是最快的：

SELECT $$SELECT sum(user_count) AS user_count_sum
      ,sum(min_count)  AS min_count_sum
      ,sum(max_count)  AS max_count_sum
FROM  (
   ($$
||
string_agg(format($$
   SELECT max(case when di = 'i' then di_count end) AS user_count
         ,min(di_count) AS min_count
         ,max(di_count) AS max_count
   FROM  (
      SELECT di, count(distinct rt) AS di_count
      FROM   %I
      WHERE  rt >  now() - interval '1 day'
      GROUP  BY 1
      ) t
   )$$, tbl)
      ,'

   UNION ALL
   (')
|| '
   ) sub'
FROM   mastertable;

产生查询字符串：

SELECT sum(user_count) AS user_count_sum
      ,sum(min_count)  AS min_count_sum
      ,sum(max_count)  AS max_count_sum
FROM  (
   (
   SELECT max(case when di = 'i' then di_count end) AS user_count
         ,min(di_count) AS min_count
         ,max(di_count) AS max_count
   FROM  (
      SELECT di, count(distinct rt) AS di_count
      FROM   gpstablev2
      WHERE  rt >  now() - interval '1 day'
      GROUP  BY 1
      ) t
   )

   UNION ALL
   (
   SELECT max(case when di = 'i' then di_count end) AS user_count
         ,min(di_count) AS min_count
         ,max(di_count) AS max_count
   FROM  (
      SELECT di, count(distinct rt) AS di_count
      FROM   powertablev2
      WHERE  rt >  now() - interval '1 day'
      GROUP  BY 1
      ) t
   )

   UNION ALL
   ...

   ) sub;

• GETDATE()不是有效的Postgres函数。 您可能是指now()或CURRENT_DATE ..，具体取决于rt的未公开类型。 我用now()代替。

• 您很可能想在这里使用UNION ALL而不是UNION 。

• 还修复了子查询缺少的别名，并做了一些小的简化。

使用EXECUTE将其包装到一个plpgsql函数中。 这里有很多与SO紧密相关的示例，它们带有代码，链接和说明...