优化器的查询转换

1.视图合并

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 / [email protected]$2
   2 - SEL$F5BB74E1 / [email protected]$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>

2.谓词推进

优化器将查询块的相关的谓词推进到视图查询块。

a.定义一个视图

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

b.执行查询

select ename
from all_emp_vw
where empno = 50;

c.谓词推进(优化器真正执行的sql)

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 );

3.非嵌套子查询：将子查询转换成等价的join连接

4.使用物化视图进行查询重写