Sql 在select语句中调优oracle子查询

我有一个主表和一个参考表，如下所示

WITH MAS as (
SELECT 10 as CUSTOMER_ID, 1 PROCESS_ID, 44 PROCESS_TYPE, 200 as AMOUNT FROM DUAL UNION ALL
SELECT 10 as CUSTOMER_ID, 1 PROCESS_ID, 44 PROCESS_TYPE, 250 as AMOUNT FROM DUAL UNION ALL
SELECT 10 as CUSTOMER_ID, 2 PROCESS_ID, 45 PROCESS_TYPE, 300 as AMOUNT FROM DUAL UNION ALL
SELECT 10 as CUSTOMER_ID, 2 PROCESS_ID, 45 PROCESS_TYPE, 350 as AMOUNT FROM DUAL 
), REFTAB as (
SELECT 44 PROCESS_TYPE, 'A' GROUP_ID FROM DUAL UNION ALL 
SELECT 44 PROCESS_TYPE, 'B' GROUP_ID FROM DUAL UNION ALL
SELECT 45 PROCESS_TYPE, 'C' GROUP_ID FROM DUAL UNION ALL 
SELECT 45 PROCESS_TYPE, 'D' GROUP_ID FROM DUAL
) SELECT ...

我的第一个正确工作的

select

语句是：

SELECT CUSTOMER_ID,
       SUM(AMOUNT) as AMOUNT1,
       SUM(CASE WHEN PROCESS_TYPE IN (SELECT PROCESS_TYPE FROM REFTAB WHERE GROUP_ID = 'A') 
                THEN AMOUNT ELSE NULL END) as AMOUNT2,
       COUNT(CASE WHEN PROCESS_TYPE IN (SELECT PROCESS_TYPE FROM REFTAB WHERE GROUP_ID = 'D') 
                  THEN 1 ELSE NULL END) as COUNT1
   FROM MAS
  GROUP BY CUSTOMER_ID

但是，为了解决性能问题，我将其更改为以下

select

语句：

SELECT CUSTOMER_ID,
       SUM(AMOUNT) as AMOUNT1,
       SUM(CASE WHEN GROUP_ID = 'A' THEN AMOUNT ELSE NULL END) as AMOUNT2,
       COUNT(CASE WHEN GROUP_ID = 'D' THEN 1 ELSE NULL END) as COUNT1
   FROM MAS A
   LEFT JOIN REFTAB B ON A.PROCESS_TYPE = B.PROCESS_TYPE
  GROUP BY CUSTOMER_ID

对于

AMOUNT2

和

COUNT1

列，值保持不变。但是对于

AMOUNT1

，由于与引用表的联接，该值会成倍增加

我知道我可以在

GROUP\u ID

上添加一个附加连接条件，再添加一个左连接。但这与使用子查询没有什么不同

---

你知道如何在不乘以

AMOUNT1

值的情况下，只使用1个左连接进行查询吗？

通常的方法是将

分组之前的值进行聚合。如果查询的其余部分正确，还可以使用条件聚合：

SELECT CUSTOMER_ID,
       SUM(CASE WHEN seqnum = 1 THEN AMOUNT END) as AMOUNT1,
       SUM(CASE WHEN GROUP_ID = 'A' THEN AMOUNT ELSE NULL END) as AMOUNT2,
       COUNT(CASE WHEN GROUP_ID = 'D' THEN 1 ELSE NULL END) as COUNT1
FROM MAS A LEFT JOIN
     (SELECT B.*, ROW_NUMBER() OVER (PARTITION BY PROCESS_TYPE ORDER BY PROCESS_TYPE) as seqnum
      FROM REFTAB B
     ) B
     ON A.PROCESS_TYPE = B.PROCESS_TYPE
GROUP BY CUSTOMER_ID;

这将忽略连接创建的重复项

我知道我可以通过添加传统的GROUP_ID子句再添加1个左连接，但它与子查询没有什么不同

你会感到惊讶的。在
SELECT
中使用2个左连接而不是子查询，可以为优化器提供更多优化查询的方法。我仍然会尝试：

select m.customer_id,
       sum(m.amount) as amount1,
       sum(case when grpA.group_id is not null then m.amount end) as amount2,
       count(grpD.group_id) as count1
  from mas m
  left join reftab grpA
    on grpA.process_type = m.process_type
   and grpA.group_id = 'A'
  left join reftab grpD
    on grpD.process_type = m.process_type
   and grpD.group_id = 'D'
 group by m.customer_id

您也可以尝试此查询，它使用
SUM（）
分析函数计算联接前的
amount1
值，以避免重复值问题：

select m.customer_id,
       m.customer_sum as amount1,
       sum(case when r.group_id = 'A' then m.amount end) as amount2,
       count(case when r.group_id = 'D' then 'X' end) as count1
  from (select customer_id,
               process_type,
               amount,
               sum(amount) over (partition by customer_id) as customer_sum
          from mas) m
  left join reftab r
    on r.process_type = m.process_type
 group by m.customer_id,
          m.customer_sum

您可以测试这两个选项，看看哪一个执行得更好。
从原始查询开始，简单地用
EXISTS
语句替换
查询中的
，应该可以显著提高性能。另外，要小心求和
NULL
s，也许你的
ELSE
语句应该是
0

SELECT CUSTOMER_ID,
       SUM(AMOUNT) as AMOUNT1,
       SUM(CASE WHEN EXISTS(SELECT 1 FROM REFTAB WHERE REFTAB.GROUP_ID = 'A' AND REFTAB.PROCESS_TYPE = MAS.PROCESS_TYPE)
                THEN AMOUNT ELSE NULL END) as AMOUNT2,
       COUNT(CASE WHEN EXISTS(SELECT 1 FROM REFTAB WHERE REFTAB.GROUP_ID = 'D' AND REFTAB.PROCESS_TYPE = MAS.PROCESS_TYPE) 
                  THEN 1 ELSE NULL END) as COUNT1
   FROM MAS
  GROUP BY CUSTOMER_ID