full join 太费时间了
需求：将表 A,B,C 合并到一个结果集中
表A如图：
N D
1 eeee
3 dddd
5 cccc
7 bbbb
9 aaaa
11 dddd
13 eeee
15 wwww
17 qqqq
19 tttttt表B如图：
N E
5 rrrrrr
4 fffff
3 ssssss
2 jjjjjj
1 kkkkkk
7 uuuuuu表C如图：
N F
5 oooo
4 lllll
3 hhss
2 ddfj
1 kdsfkkk
7 sduuu
8 ewrtwy
12 sdgfsd
22 dfgee要得到的结果集如图：
N D E F
1 eeee kkkkkk kdsfkkk
22 dfgee
11 dddd
13 eeee
2 jjjjjj ddfj
5 cccc rrrrrr oooo
4 fffff lllll
17 qqqq
8 ewrtwy
3 dddd ssssss hhss
7 bbbb uuuuuu sduuu
9 aaaa
15 wwww
19 tttttt
12 sdgfsd两种思路：
1, full join
语句为：
select nvl(a.n,nvl(b.n,c.n)) as N,
a.d,b.e,c.f
from A full join B on A.N=B.N
full join c on b.n=c.n;2, 先union all ，再列转行
语句为：
select n ,max(case when nn='a' then d end) as d,
max(case when nn='b' then d end) as e,
max(case when nn='c' then d end) as f
from (
select n,d as d,'a' as nn from a
union all
select n,e as d,'b' as nn from b
union all
select n,f as d,'c' as nn from c
)
group by n;使用 full join 的语句
explain plan for
select nvl(a.n,nvl(b.n,c.n)) as N,
a.d,b.e,c.f
from A full join B on A.N=B.N full join c on b.n=c.n;
select * from table(dbms_xplan.display());执行结果为
Plan hash value: 2877137913
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 19 | 1197 | 122 (4)| 00:00:02 |
| 1 | VIEW | | 19 | 1197 | 122 (4)| 00:00:02 |
| 2 | UNION-ALL | | | | | |
|* 3 | HASH JOIN OUTER | | 11 | 1342 | 61 (4)| 00:00:01 |
| 4 | VIEW | | 11 | 825 | 57 (2)| 00:00:01 |
| 5 | UNION-ALL | | | | | |
|* 6 | HASH JOIN OUTER | | 10 | 60 | 29 (4)| 00:00:01 |
| 7 | TABLE ACCESS FULL| A | 10 | 30 | 14 (0)| 00:00:01 |
| 8 | TABLE ACCESS FULL| B | 5 | 15 | 14 (0)| 00:00:01 |
|* 9 | HASH JOIN ANTI | | 1 | 6 | 29 (4)| 00:00:01 |
| 10 | TABLE ACCESS FULL| B | 5 | 15 | 14 (0)| 00:00:01 |
| 11 | TABLE ACCESS FULL| A | 10 | 30 | 14 (0)| 00:00:01 |
| 12 | TABLE ACCESS FULL | C | 9 | 423 | 3 (0)| 00:00:01 |
|* 13 | HASH JOIN ANTI | | 8 | 272 | 61 (4)| 00:00:01 |
| 14 | TABLE ACCESS FULL | C | 9 | 189 | 3 (0)| 00:00:01 |
| 15 | VIEW | | 11 | 143 | 57 (2)| 00:00:01 |
| 16 | UNION-ALL | | | | | |
|* 17 | HASH JOIN OUTER | | 10 | 60 | 29 (4)| 00:00:01 |
| 18 | TABLE ACCESS FULL| A | 10 | 30 | 14 (0)| 00:00:01 |
| 19 | TABLE ACCESS FULL| B | 5 | 15 | 14 (0)| 00:00:01 |
|* 20 | HASH JOIN ANTI | | 1 | 6 | 29 (4)| 00:00:01 |
| 21 | TABLE ACCESS FULL| B | 5 | 15 | 14 (0)| 00:00:01 |
| 22 | TABLE ACCESS FULL| A | 10 | 30 | 14 (0)| 00:00:01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("B"."N"="C"."N"(+))
6 - access("A"."N"="B"."N"(+))
9 - access("A"."N"="B"."N")
13 - access("B"."N"="C"."N")
17 - access("A"."N"="B"."N"(+))
20 - access("A"."N"="B"."N")
Note
-----
- dynamic sampling used for this statement使用union all 的语句
explain plan for
select n ,max(case when nn='a' then d end) as d,
max(case when nn='b' then d end) as e,
max(case when nn='c' then d end) as f
from (
select n,d as d,'a' as nn from a
union all
select n,e as d,'b' as nn from b
union all
select n,f as d,'c' as nn from c
)
group by n;
select * from table(dbms_xplan.display());执行结果为：
1 Plan hash value: 1237158055
2
3 -----------------------------------------------------------------------------
4 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
5 -----------------------------------------------------------------------------
6 | 0 | SELECT STATEMENT | | 24 | 576 | 32 (4)| 00:00:01 |
7 | 1 | HASH GROUP BY | | 24 | 576 | 32 (4)| 00:00:01 |
8 | 2 | VIEW | | 24 | 576 | 31 (0)| 00:00:01 |
9 | 3 | UNION-ALL | | | | | |
10 | 4 | TABLE ACCESS FULL| A | 10 | 30 | 14 (0)| 00:00:01 |
11 | 5 | TABLE ACCESS FULL| B | 5 | 15 | 14 (0)| 00:00:01 |
12 | 6 | TABLE ACCESS FULL| C | 9 | 189 | 3 (0)| 00:00:01 |
13 -----------------------------------------------------------------------------
14
15 Note
16 -----
17 - dynamic sampling used for this statement对比2种处理方式，union all 的方式明显优于 full join。也可以看出简单的sql语句效率不一定好。