oracle连接中on (1=1)

大家：

好！最近看了oracle连接中的on(1=1)的问题，挺有意思的。

测试表如下所示:

create table t1(c1 varchar2(10));
insert into t1 values('1');
insert into t1 values('2');


create table t2(c2 varchar2(10));
insert into t2 values('1');

简简单单的一个表，最多两条数据, 问下面的查询会出什么结果？

SELECT T1.C1, T2.C2, T3.C3
  FROM T1
  JOIN T2
    ON (1 = 1)
  LEFT JOIN (SELECT 1 AS C3 FROM DUAL UNION ALL SELECT 2 FROM DUAL) T3
    ON (T1.C1 = T2.C2)

直接看执行计划

SQL> SELECT T1.C1, T2.C2, T3.C3
  2    FROM T1
  3    JOIN T2
  4      ON (1 = 1)
  5    LEFT JOIN (SELECT 1 AS C3 FROM DUAL UNION ALL SELECT 2 FROM DUAL) T3
  6      ON (T1.C1 = T2.C2);


执行计划
----------------------------------------------------------                                          
Plan hash value: 769723218                                                                          
                                                                                                    
------------------------------------------------------------------------------                      
| Id  | Operation             | Name | Rows  | Bytes | Cost (%CPU)| Time     |                      
------------------------------------------------------------------------------                      
|   0 | SELECT STATEMENT      |      |     4 |    68 |    14   (0)| 00:00:01 |                      
|   1 |  NESTED LOOPS OUTER   |      |     4 |    68 |    14   (0)| 00:00:01 |                      
|   2 |   MERGE JOIN CARTESIAN|      |     2 |    28 |     6   (0)| 00:00:01 |                      
|   3 |    TABLE ACCESS FULL  | T2   |     1 |     7 |     3   (0)| 00:00:01 |                      
|   4 |    BUFFER SORT        |      |     2 |    14 |     3   (0)| 00:00:01 |                      
|   5 |     TABLE ACCESS FULL | T1   |     2 |    14 |     3   (0)| 00:00:01 |                      
|   6 |   VIEW                |      |     2 |     6 |     4   (0)| 00:00:01 |                      
|*  7 |    FILTER             |      |       |       |            |          |                      
|   8 |     VIEW              |      |     2 |     6 |     4   (0)| 00:00:01 |                      
|   9 |      UNION-ALL        |      |       |       |            |          |                      
|  10 |       FAST DUAL       |      |     1 |       |     2   (0)| 00:00:01 |                      
|  11 |       FAST DUAL       |      |     1 |       |     2   (0)| 00:00:01 |                      
------------------------------------------------------------------------------                      
                                                                                                    
Predicate Information (identified by operation id):                                                 
---------------------------------------------------                                                 
                                                                                                    
   7 - filter("T1"."C1"="T2"."C2")                                                                  
                                                                                                    
Note                                                                                                
-----                                                                                               
   - dynamic sampling used for this statement (level=2)                                             


统计信息
----------------------------------------------------------                                          
          0  recursive calls                                                                        
          0  db block gets                                                                          
         14  consistent gets                                                                        
          0  physical reads                                                                         
          0  redo size                                                                              
        583  bytes sent via SQL*Net to client                                                       
        416  bytes received via SQL*Net from client                                                 
          2  SQL*Net roundtrips to/from client                                                      
          1  sorts (memory)                                                                         
          0  sorts (disk)                                                                           
          3  rows processed

个人的理解：1 从id=2 中可以看到,t1和t2是做了笛卡尔积
2 id=7 中可以看到,关联t3的条件 filter("T1"."C1"="T2"."C2") 有点不好理解，但确实是符合语法

先验证下，我的id=2的猜想是否正确

可以看到笛卡尔积的猜想是对的，最终的结果如下：

可以看到,是对关联后的"1,1"又进行了一次笛卡尔积，因为关联条件中没有t3的限制条件。

结果中会有第三行，是因为关联t3用的是left join。如果换成内连接,是不是第三行就没有了？

结果验证正确

oracle连接中on (1=1)

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