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HomeDatabaseMysql Tutorial关于Oracle full outer join 的bug问题分析及处理

full (outer) join是用来全连接两个表的语法。即希望将A表和B表关联,能够得到A表中有而B表中没有的记录,或者B表中有而A表中没有

full (outer) join是用来全连接两个表的语法。即希望将A表和B表关联,能够得到A表中有而B表中没有的记录,或者B表中有而A表中没有的记录。

如何判断是否有该记录,则通过on子句来关联。

下面是一个例子:

SQL> with
  2  A as(select 1 a, 2 b from dual),
  3  B as(select 2 a, 3 b from dual)
  4  select * from A full join B
  5      on A.a=B.a
  6  /
 
        A          B          A          B
---------- ---------- ---------- ----------
        1          2         
                                  2          3
 

了解了以上基本原理后,我们应该知道,理论上讲,A表和B表的在from子句中的顺序是没有关系的,也就是不影响结果。但是,实际上,却出现了这样的问题,下面是对这种情况的描述:

--------------------------------------------------------------------------------------------
| Id  | Operation                | Name            | Rows  | Bytes | Cost (%CPU)| Time    |
--------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT        |                | 12791 |  349K|    82  (3)| 00:00:01 |
|  1 |  VIEW                    |                | 12791 |  349K|    82  (3)| 00:00:01 |
|  2 |  UNION-ALL              |                |      |      |            |          |
|*  3 |    FILTER                |                |      |      |            |          |
|*  4 |    HASH JOIN RIGHT OUTER|                | 12790 |  1124K|    41  (3)| 00:00:01 |
|  5 |      TABLE ACCESS FULL  | JXC_RISHARESUM  |  1735 | 78075 |    7  (0)| 00:00:01 |
|  6 |      TABLE ACCESS FULL  | JXC_ALLTRADEDAY | 12790 |  562K|    33  (0)| 00:00:01 |
|*  7 |    HASH JOIN ANTI        |                |    1 |    76 |    41  (3)| 00:00:01 |
|*  8 |    TABLE ACCESS FULL    | JXC_RISHARESUM  |    1 |    45 |    7  (0)| 00:00:01 |
|  9 |    TABLE ACCESS FULL    | JXC_ALLTRADEDAY | 12790 |  387K|    33  (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

 

从以上执行计划来看,在第四步骤,,使用的是hash join rigth outer连接方式。而通过改变两表的摆放顺序,得到如下的执行计划:

 

-----------------------------------------------------------------------------------------
| Id  | Operation            | Name            | Rows  | Bytes | Cost (%CPU)| Time    |
-----------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT      |                |  1876 | 52528 |    82  (3)| 00:00:01 |
|  1 |  VIEW                |                |  1876 | 52528 |    82  (3)| 00:00:01 |
|  2 |  UNION-ALL          |                |      |      |            |          |
|*  3 |    FILTER            |                |      |      |            |          |
|*  4 |    HASH JOIN OUTER  |                |  1874 |  164K|    41  (3)| 00:00:01 |
|  5 |      TABLE ACCESS FULL| JXC_RISHARESUM  |  1735 | 78075 |    7  (0)| 00:00:01 |
|  6 |      TABLE ACCESS FULL| JXC_ALLTRADEDAY | 12790 |  562K|    33  (0)| 00:00:01 |
|*  7 |    HASH JOIN ANTI    |                |    2 |  152 |    41  (3)| 00:00:01 |
|*  8 |    TABLE ACCESS FULL | JXC_ALLTRADEDAY |    2 |    90 |    33  (0)| 00:00:01 |
|  9 |    TABLE ACCESS FULL | JXC_RISHARESUM  |  1735 | 53785 |    7  (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

 

注意,此时,执行计划中的第四个步骤,变成了:hash join outer方式。这个才是我们所期望的方式。那究竟是什么导致了这个变化呢?查看他们的谓词连接逻辑:

hash join right outer的:

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