-1

假设我有下表：

create table Employee
  (
  ID                 VARCHAR2(4 BYTE)         NOT NULL,
  First_Name         VARCHAR2(10 BYTE),
  Last_Name          VARCHAR2(10 BYTE),
  Start_Date         DATE,
  End_Date           DATE,
  Salary             Number(8,2),
  City               VARCHAR2(10 BYTE),
  Description        VARCHAR2(15 BYTE)
  )
  ;

insert into employee (
ID,  First_Name, Last_Name, Start_Date,                               
End_Date,                       Salary,  City,       Description)
select 21 + (level * 2) - 1,'Jason',    'Martin',  to_date('19960725','YYYYMMDD'), 
to_date ('20060725','YYYYMMDD'), 1234.56+level/3*5, 'Toronto',  'Programmer'
from dual
connect by level < 2501
;

当我尝试运行以下查询时：

with tt2 as (
select t.*, 
case when rownum = 1 then 1 when rownum = 3 then 3 else null end rfn

-- RFN 是一个复杂情况的例子，它只为需要的行分配数字。请不要修复它

from (select id, first_name, salary from employee) t
)

select  
(select id from tt2 where rfn = 3) w
from dual

它返回给我所有类似于 rfn = 3 的 ID（2500 行，共 26 行）

这是解释计划

"PLAN_TABLE_OUTPUT"
"Plan hash value: 2716438026"
" "
"--------------------------------------------------------------------------------------------------------"
"| Id  | Operation                  | Name                      | Rows  | Bytes | Cost (%CPU)| Time     |"
"--------------------------------------------------------------------------------------------------------"
"|   0 | SELECT STATEMENT           |                           |  2500 |       |    12   (0)| 00:00:01 |"
"|*  1 |  VIEW                      |                           |  2500 | 42500 |     4   (0)| 00:00:01 |"
"|   2 |   TABLE ACCESS FULL        | SYS_TEMP_0FD9D6645_8CDDE2 |  2500 | 60000 |     4   (0)| 00:00:01 |"
"|   3 |  TEMP TABLE TRANSFORMATION |                           |       |       |            |          |"
"|   4 |   LOAD AS SELECT           | SYS_TEMP_0FD9D6645_8CDDE2 |       |       |            |          |"
"|   5 |    COUNT                   |                           |       |       |            |          |"
"|   6 |     TABLE ACCESS FULL      | EMPLOYEE                  |  2500 | 60000 |     8   (0)| 00:00:01 |"
"|   7 |   VIEW                     |                           |  2500 |       |     4   (0)| 00:00:01 |"
"|   8 |    TABLE ACCESS FULL       | SYS_TEMP_0FD9D6645_8CDDE2 |  2500 | 60000 |     4   (0)| 00:00:01 |"
"--------------------------------------------------------------------------------------------------------"
" "
"Predicate Information (identified by operation id):"
"---------------------------------------------------"
" "
"   1 - filter(""RFN""=3)"
" "
"Note"
"-----"
"   - dynamic sampling used for this statement (level=2)"

- 编辑 -

例如，我在 WITH 中有表：

"ID" "FIRST_NAME" "SALARY" "RFN"
“22”“杰森”1236.23 1
“24”“杰森”1237.89
“26”“杰森”1239.56 3

我想选择 RFN = 1 的任何人的薪水，给我一个号码 1236.23

但我也想在同一个查询中选择 RFN = 3 的薪水，给我一个号码 1239.56。

等等。

但。对于此查询中的每一个，它都会生成 N 行的 FULL_TABLE_SCAN。

with tt2 as (
select t.*, case when rownum = 1 then 1 when rownum = 3 then 3 else null end rfn
from (select id, first_name, salary from employee) t
)
select  
(select salary from tt2 where rfn = 1) w,
(select salary from tt2 where rfn = 3) w
from dual

解释表：

('Plan hash value: 1823688231');
(' ');
('--------------------------------------------------------------------------------------------------------');
('| Id  | Operation                  | Name                      | Rows  | Bytes | Cost (%CPU)| Time     |');
('--------------------------------------------------------------------------------------------------------');
('|   0 | SELECT STATEMENT           |                           |  2500 |       |    12   (0)| 00:00:01 |');
('|*  1 |  VIEW                      |                           |  2500 | 40000 |     4   (0)| 00:00:01 |');
('|   2 |   TABLE ACCESS FULL        | SYS_TEMP_0FD9D6651_8CDDE2 |  2500 | 60000 |     4   (0)| 00:00:01 |');
('|*  3 |  VIEW                      |                           |  2500 | 40000 |     4   (0)| 00:00:01 |');
('|   4 |   TABLE ACCESS FULL        | SYS_TEMP_0FD9D6651_8CDDE2 |  2500 | 60000 |     4   (0)| 00:00:01 |');
('|   5 |  TEMP TABLE TRANSFORMATION |                           |       |       |            |          |');
('|   6 |   LOAD AS SELECT           | SYS_TEMP_0FD9D6651_8CDDE2 |       |       |            |          |');
('|   7 |    COUNT                   |                           |       |       |            |          |');
('|   8 |     TABLE ACCESS FULL      | EMPLOYEE                  |  2500 | 60000 |     8   (0)| 00:00:01 |');
('|   9 |   VIEW                     |                           |  2500 |       |     4   (0)| 00:00:01 |');
('|  10 |    TABLE ACCESS FULL       | SYS_TEMP_0FD9D6651_8CDDE2 |  2500 | 60000 |     4   (0)| 00:00:01 |');
('--------------------------------------------------------------------------------------------------------');
(' ');
('Predicate Information (identified by operation id):');
('---------------------------------------------------');
(' ');
('   1 - filter("RFN"=1)');
('   3 - filter("RFN"=3)');
(' ');
('Note');
('-----');
('   - dynamic sampling used for this statement (level=2)');

——为更清楚而编辑——

预期结果来自：

"ID" "FIRST_NAME" "SALARY" "RFN"
“22”“杰森”1236.23 1
“24”“杰森”1237.89
“26”“杰森”1239.56 3
“26”“杰克”1249.56 6
“26”“维基”119.56 8

工资 工资 1 工资 2 姓名 姓名 2
1236.23 1249.56 119.56 维基杰克

来自〜以下声明：

with tt2 as (
select t.*, case when row_number() over (order by id) = 1 then 1 when row_number() over     (order by id) = 3 then 3 else null end rfn
from (select id, first_name, salary from employee) t
)
select 
(select salary from tt2 where rfn = 1) salary,
(select salary from tt2 where rfn = 6) salary1,
(select salary from tt2 where rfn = 8) salary2,
(select first_name from tt2 where rfn = 8) name,
(select first_name from tt2 where rfn = 6) name2

from dual

没有 2500 行的 5 次全表扫描

4

1 回答 1

0

您要求所有行，您必须更改为：

with tt2 as (
select t.*, case when rownum = 1 then 1 when rownum = 3 then 3 else null end rfn
from (select id, first_name, salary from employee) t
)
select salary from tt2 where rfn in (1,3)

但是为什么要这个结果呢？如果没有 ORDER BY，ROWNUM 就不是确定性的，不能保证每次都具有相同的顺序。您可能需要像这样使用 ROW_NUMBER：

WITH tt2 AS (SELECT t.*, ROW_NUMBER() OVER (ORDER BY ID) rn)
SELECT salary 
FROM tt2 
WHERE rn IN (1,3)
于 2013-09-15T11:40:19.030 回答