优化器的查询转换

SQL> create view emp_vm as
2  select empno,ename,job,sal,comm,deptno
3  from emp
4  where deptno=30;

View created.

SQL> select empno
2  from emp_vm
3  where empno > 150;

EMPNO
----------
7499
7521
7654
7698
7844
7900

6 rows selected.

SQL> select * from table(dbms_xplan.display_cursor(null, null, 'advanced'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  997u1v1b1k5sc, child number 0
-------------------------------------
select empno from emp_vm where empno > 150

Plan hash value: 169057108

--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |       |       |     2 (100)|          |
|*  1 |  TABLE ACCESS BY INDEX ROWID| EMP    |     5 |    35 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | PK_EMP |    14 |       |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$F5BB74E1 / EMP@SEL$2
2 - SEL$F5BB74E1 / EMP@SEL$2

Outline Data
-------------

/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('11.2.0.4')
DB_VERSION('11.2.0.4')
ALL_ROWS
OUTLINE_LEAF(@"SEL$F5BB74E1")
MERGE(@"SEL$2")
OUTLINE(@"SEL$1")
OUTLINE(@"SEL$2")
INDEX_RS_ASC(@"SEL$F5BB74E1" "EMP"@"SEL$2" ("EMP"."EMPNO"))
END_OUTLINE_DATA
*/

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("DEPTNO"=30)
2 - access("EMPNO">150)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

1 - "EMPNO"[NUMBER,22]
2 - "EMP".ROWID[ROWID,10], "EMPNO"[NUMBER,22]

49 rows selected.

SQL>

create view all_emp_vw as
( select empno, ename, job, comm, deptno from emp )
union
( select empno, ename, job, comm, deptno from contract_workers );

select ename
from all_emp_vw
where empno = 50;

select ename
from ( select empno, ename, job, comm, deptno from emp where empno=50
union
select empno, ename, job, comm, deptno from contract_workers where empno=50 );