执行计划-CONCATENATION释疑

SQL> set linesize 200
SQL> set pagesize 20000
SQL> set autotrace trace
SQL> create table dao_objects as select  * from dba_objects ;


Table created.

SQL> create index idx_obj_id on dao_objects (object_id) ;
create index  idx_obj_type on dao_objects (object_type) ;


Index created.


SQL> 
Index created.




SQL>  select *                                                                               
  2   from dao_objects do                                                                    
  3   where do.object_id<2000                                                                
  4    or do.object_type='VIEW';                                                             
                                                                                             
                                                                                             
Execution Plan                                                                               
----------------------------------------------------------                                   
Plan hash value: 3924143853                                                                  
                                                                                             
---------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |  7479 |  1511K|    36   (0)| 00:00:01 |
|   1 |  CONCATENATION               |              |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| DAO_OBJECTS  |  5181 |  1047K|    20   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | IDX_OBJ_TYPE |   415 |       |     5   (0)| 00:00:01 |
|*  4 |   TABLE ACCESS BY INDEX ROWID| DAO_OBJECTS  |  2298 |   464K|    16   (0)| 00:00:01 |
|*  5 |    INDEX RANGE SCAN          | IDX_OBJ_ID   |   934 |       |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
                                                                                             
Predicate Information (identified by operation id):                                          
---------------------------------------------------                                          
                                                                                             
   3 - access("DO"."OBJECT_TYPE"='VIEW')                                                     
   4 - filter(LNNVL("DO"."OBJECT_TYPE"='VIEW'))                                              
   5 - access("DO"."OBJECT_ID"<2000)                                                         
                                                                                             
Note                                                                                         
-----                                                                                        
   - dynamic sampling used for this statement (level=2)                                      

请注意ID为1的操作CONCATENATION.


CONCATENATION操作类似 union ,只不过与union 不同的是他并不对全部数据去重.
CONCATENATION操作只去除由OR引起的重复值.这点在我们做SQL改写的时候一定要特别注意.
因为empno为7934的记录 部门编号为20

SQL> select e.empno,e.deptno
  2    from scott.emp e 
  3   where empno >7900 or deptno=10 ;


     EMPNO     DEPTNO
---------- ----------
      7782         10
      7839         10
      7902         20
      7934         10


SQL> select e.empno,e.deptno
  2    from scott.emp e 
  3   where empno >7900 ;


     EMPNO     DEPTNO
---------- ----------
      7902         20
      7934         10


SQL> select e.empno,e.deptno
  2    from scott.emp e 
  3   where deptno=10;


     EMPNO     DEPTNO
---------- ----------
      7782         10
      7839         10
      7934         10