组合SQL语句以优化

Question

我在同一表中有2个不同的查询。第一个是形式 -

SELECT MIN(A) AS MIN_A, MAX(B) AS MAX_B
FROM myTable
WHERE timeStamp > ?
GROUP BY type;

.

和另一个：

SELECT SUM (CASE WHEN io > 0 THEN 1 ELSE 0 END) as io_cnt
FROM (
     SELECT
     (CASE WHEN SUM(io_ops) > 0 THEN 1 ELSE 0 END) as io
     FROM myTable
     WHERE timestamp > ? AND type = ?
     GROUP BY id
) t;

.

和表有列 - a，b，ID，时间戳，类型。 现在我呼唤java的第一个查询，取出结果集输出并循环，呼叫第一个查询中返回的每个类型的第二个查询。

我需要两个min（a），首先查询的MAX（B），并且IO计数来自第二个查询。是否有可能在一个查询中进行？我正在使用Amazon Redshift作为我的数据库。

Answer 1

redshift是相当限的。它基于PostgreSQL 8.0.2，不支持许多新功能。这应该是工作（未经测试）：

SELECT t.type, min(min_a) AS min_a, max(max_b) AS max_b
      ,count(io > 0 OR NULL) AS io_cnt
FROM  (
   SELECT type, min(a) as min_a, max(b) as max_b
         ,sum(io_ops) AS io
   FROM   myTable
   WHERE  timestamp > ?
   GROUP  BY type, id
   ) t
GROUP  BY t.type;

.

取决于数据分布，这可能会更快或更快：

SELECT t.type, m.min_a, m.max_b, count(io > 0 OR NULL) AS io_cnt
FROM  (
   SELECT type, sum(io_ops) AS io
   FROM   myTable
   WHERE  timestamp > ?
   GROUP  BY type, id
   ) t
JOIN  (
   SELECT type, min(a) as min_a, max(b) as max_b
   FROM   myTable
   WHERE  timeStamp > ?
   GROUP  BY type
   ) m USING (type)
GROUP  BY  t.type, m.min_a, m.max_b;

.