如何在 Oracle 中使用 max in where 子句优化查询

Question

我提供假设 table 和查询来解释我的问题。请原谅我的语法错误。

部门 table:

ID Dno Dname BDate   seq
1  1     A    5-Aug   0
2  1     B    3-Aug   0
3  1     B    7-Aug   1
4  1     C    2-Aug   0

下方查询returns 1st, and 3rd, record from above table:

select * from Dept where BDate > (select mydate from other_table)
-- mydate is 4-Aug

然后我在查询中对 return 第二条记录进行了以下更改。因为对于 Dname 'B'，我们有一条 Bdate > 4-Aug.

的记录

select * from Dept D where
(SELECT MAX(BDATE)
FROM Dept D1
WHERE D1.Dno = D.Dno
AND D1.Dname = D.Dname
) > (select mydate from other_table)

以上查询有效，但影响了性能。我该如何优化它。

我想到联合或将最大查询移动到 select 部分。但是，找不到方法。

Answer 1

假设我正确理解您的逻辑（如果最大日期大于指定日期，您需要给定 dno 和 dname 的所有行）并且查询检索 "mydate" returns 一行，我会做类似的事情：

with     dept as (select 1 id, 1 dno, 'A' dname, to_date('05/08/2015', 'dd/mm/yyyy') bdate, 0 seq from dual union all
                  select 2 id, 1 dno, 'B' dname, to_date('03/08/2015', 'dd/mm/yyyy') bdate, 0 seq from dual union all
                  select 3 id, 1 dno, 'B' dname, to_date('07/08/2015', 'dd/mm/yyyy') bdate, 0 seq from dual union all
                  select 4 id, 1 dno, 'C' dname, to_date('02/08/2015', 'dd/mm/yyyy') bdate, 0 seq from dual),
  other_table as (select to_date('04/08/2015', 'dd/mm/yyyy') mydate from dual)
select id,
       dno,
       dname,
       bdate,
       seq
from   (select d.*,
               max(bdate) over (partition by dno, dname) max_bdate
        from   dept d)
where  max_bdate > (select mydate from other_table);

        ID        DNO DNAME BDATE             SEQ
---------- ---------- ----- ---------- ----------
         1          1 A     05/08/2015          0
         2          1 B     03/08/2015          0
         3          1 B     07/08/2015          0

Answer 2

使用 group-by 子句避免其中一个子查询：

select Dno, Dname, max(BDate)
from Dept
group by Dno, Dname
having Max(BDate) > (select mydate from other_table)

您还可以使用本地变量删除其他子查询：

declare @mydate Date = (select mydate from other_table);
select Dno, Dname, max(BDate)
from Dept
group by Dno, Dname
having Max(BDate) > (@mydate)

Answer 3

如果您希望此查询 return 您 DEPT table 中的一小部分行，这对您来说可能会更快。我假设 DEPT.DNAME 是唯一的并且上面有一个索引。（当然 DEPT.BDATE 上需要有一个索引！）

with     dept as (select 1 id, 1 dno, 'A' dname, to_date('05/08/2015', 'dd/mm/yyyy') bdate, 0 seq from dual union all
                  select 2 id, 1 dno, 'B' dname, to_date('03/08/2015', 'dd/mm/yyyy') bdate, 0 seq from dual union all
                  select 3 id, 1 dno, 'B' dname, to_date('07/08/2015', 'dd/mm/yyyy') bdate, 0 seq from dual union all
                  select 4 id, 1 dno, 'C' dname, to_date('02/08/2015', 'dd/mm/yyyy') bdate, 0 seq from dual),
  other_table as (select to_date('04/08/2015', 'dd/mm/yyyy') mydate from dual)
select id,
       dno,
       dname,
       bdate,
       seq
from   dept WHERE dname IN ( SELECT d2.dname FROM dept d2 WHERE d2.bdate >  (select mydate from other_table) );

Answer 4

以下查询运行速度快 4 倍且结果正确：

select d1.* from Dept d1,
(select dno, dname, MAX(BDATE) as maxdate from Dept group by dno, dname) d2
where
d1.dno=d2.dno and d1.dname=d2.dname
and d2.maxdate > (select mydate from other_table)

Answer 5

根据数据集的选择性，可能值得尝试这种方法：

select *
from   dept
where  (dno, dname) in (
         select distinct dno, dname
         from   dept
         where  BDATE > (select mydate from other_table))

有了 dept(bdate) 和 dept(dno,dname) 上的索引，并且只匹配很少的记录，这会非常快。

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