关于查询转换的一些简单分析(三) (r3笔记第69天)

关于查询转换,已经讨论了视图合并和子查询解嵌套，还有谓词推进和物化视图查询重写也是查询转换中不可或缺的部分。 -->谓词推进 这个术语听起来高大上，有点故弄玄虚的味道。其实在我们的查询中已经潜移默化的使用到了。比如下面的这个查询。我们定义了一个子查询 v，然后在外部的过滤条件中加入了过滤条件e.deptno=10 因为e.deptno=v.deptno 所以过滤条件也可以理解为v.depetno=10

select e.ename,e.sal,v.avg_salary
from emp  e,
(select deptno,avg(sal) avg_salary
from emp oe
group by  deptno)v
where e.deptno=v.deptno and e.sal>1000
and e.deptno=10

Execution  Plan
----------------------------------------------------------
Plan hash  value: 269884559
-----------------------------------------------------------------------------
|  Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time      |
-----------------------------------------------------------------------------
|    0 | SELECT STATEMENT     |      |     3 |   117 |     5  (20)| 00:00:01 |
|*   1 |  HASH JOIN           |      |     3 |   117 |     5  (20)| 00:00:01 |
|    2 |   VIEW               |      |     1 |    26 |     2   (0)| 00:00:01 |
|    3 |    HASH GROUP BY     |      |     1 |     7 |     2   (0)| 00:00:01 |
|*   4 |     TABLE ACCESS FULL| EMP  |     3 |    21 |     2   (0)| 00:00:01 |
|*   5 |   TABLE ACCESS FULL  | EMP  |     3 |    39 |     2   (0)| 00:00:01  |
-----------------------------------------------------------------------------
Predicate Information (identified by operation  id):
---------------------------------------------------
 1 - access("E"."DEPTNO"="V"."DEPTNO")
   4 - filter("DEPTNO"=10)
    5 - filter("E"."DEPTNO"=10 AND "E"."SAL">1000)

从上面的执行计划可以看到执行计划第2,3,4行是子查询v 相关的，第4行中的过滤条件，直接在子查询中嵌入了过滤条件depetno=10。这种情况就是使用了谓词推进。谓词推进在查询中都是自动调用的。但是如果碰到了rownum,算是它的死穴。我们来看看使用了rownum之后的执行计划。

select  e.ename,e.sal,v.avg_salary
from emp e,
(select deptno,avg(sal)  avg_salary
from emp oe where rownum<100
group by deptno  )v
where e.deptno=v.deptno and e.sal>1000
and e.deptno=10

Execution  Plan
----------------------------------------------------------
Plan hash  value: 1823415238
------------------------------------------------------------------------------
|  Id  | Operation             | Name | Rows  | Bytes | Cost (%CPU)| Time      |
------------------------------------------------------------------------------
|    0 | SELECT STATEMENT      |      |     9 |   351 |     6  (34)| 00:00:01  |
|*  1 |  HASH JOIN            |      |     9 |   351 |     6  (34)|  00:00:01 |
|*  2 |   TABLE ACCESS FULL   | EMP  |     3 |    39 |     2    (0)| 00:00:01 |
|*  3 |   VIEW                |      |     3 |    78 |     3   (34)| 00:00:01 |
|   4 |    HASH GROUP BY      |      |     3 |    21 |      3  (34)| 00:00:01 |
|*  5 |     COUNT STOPKEY     |      |       |        |            |          |
|   6 |      TABLE ACCESS FULL| EMP  |    14 |     98 |     2   (0)| 00:00:01  |
------------------------------------------------------------------------------
Predicate Information (identified by operation  id):
---------------------------------------------------
 1 - access("E"."DEPTNO"="V"."DEPTNO")
   2 - filter("E"."DEPTNO"=10 AND  "E"."SAL">1000)
 3 -  filter("V"."DEPTNO"=10)
   5 - filter(ROWNUM<100)

这个执行计划中，因为使用了rownum，所以就没有使用到谓词推进。对于谓词推进也可以考虑使用hint push_pred来实现。 -->查询重写

对于物化视图有几个主要的两点用得比较多，查询重写和视图刷新。查询重写在查询转换中还是比较实用的。如果有海量的数据,更新频率不高。可以根据查询条件来定义物化视图，开启查询重写的功能。创建一个物化视图。

create  materialized view mv_emp enable query  rewrite 
as
select e.ename,e.sal,v.avg_salary
from emp  e,
(select deptno,avg(sal) avg_salary
from emp oe 
group by deptno  )v
where e.deptno=v.deptno and e.sal>1000
and  e.deptno=10

然后使用查询,执行计划如下：

Execution  Plan
----------------------------------------------------------
Plan hash  value: 2244303076
---------------------------------------------------------------------------------------
|  Id  | Operation                    | Name   | Rows  | Bytes | Cost (%CPU)|  Time      |
---------------------------------------------------------------------------------------
|    0 | SELECT STATEMENT             |        |     3 |    99 |     2   (0)|  00:00:01 |
|   1 |  MAT_VIEW REWRITE  ACCESS FULL| MV_EMP |     3 |    99 |     2   (0)| 00:00:01  |
---------------------------------------------------------------------------------------
Note
-----
   - dynamic sampling used for this statement (level=2)