with as 加上 materialize hint 生成实质临时表

WITH AS： 
就是将一个子查询部分独立出来，有时候是为了提高SQL语句的可读性，有时候是为了提高SQL语句性能。
如果一个SQL语句中，某个表会被访问多次，而且每次访问的限制条件一样的话，就可以使用with as来提高性能。
注意：
如果 with as 短语没有被调用2次以上，CBO就不会讲这个短语获取的数据放入temp表，如果想要讲数据放入temp表需要使用materialize hint
如果 with as 短语被调用了2次以上， CBO会自动将 with as 短语的数据放入一个临时表，这个时候不用写materialize hint

举个例子(本例基于Scott用户)

SQL> explain plan for
with a as (select /*+ materialize */ ename,job,deptno from emp where sal>(select avg(sal) from emp))
select * from a ;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------
Plan hash value: 

-------------------------------------------------------------------------------------------------------
| Id  | Operation                  | Name                     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------------
|    | SELECT STATEMENT           |                          |      |     |        ()| :: |
|    |  TEMP TABLE TRANSFORMATION |                          |       |       |            |          |
|    |   LOAD AS SELECT           | SYS_TEMP_0FD9D6605_E16CE |       |       |            |          |
|*   |    TABLE ACCESS FULL       | EMP                      |      |     |        ()| :: |
|    |     SORT AGGREGATE         |                          |      |      |            |          |
|    |      TABLE ACCESS FULL     | EMP                      |     |     |        ()| :: |
|    |   VIEW                     |                          |      |     |        ()| :: |
|    |    TABLE ACCESS FULL       | SYS_TEMP_0FD9D6605_E16CE |      |     |        ()| :: |
-------------------------------------------------------------------------------------------------------

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

    - filter("SAL"> (SELECT AVG("SAL") FROM "EMP" "EMP"))

 rows selected.

去掉 /*+ materialize */ ，由于只访问了一次a,所以CBO不会将a的查询结果生成一个临时表

SQL> explain plan for
with a as (select ename,job,deptno from emp where sal>(select avg(sal) from emp))
select * from a ;      

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------
Plan hash value: 

----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|    | SELECT STATEMENT    |      |      |     |        ()| :: |
|*   |  TABLE ACCESS FULL  | EMP  |      |     |        ()| :: |
|    |   SORT AGGREGATE    |      |      |      |            |          |
|    |    TABLE ACCESS FULL| EMP  |     |     |        ()| :: |
----------------------------------------------------------------------------

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

    - filter("SAL"> (SELECT AVG("SAL") FROM "EMP" "EMP"))

 rows selected.

WITH AS 语句调用一次 使用多次 需要写hints

如果  表 只 扫描 1次,你些materialize hints 结果读了一次 还写入temp, 再从temp读出来
临时表写入是1次,但是读要多次。

继续测试:
SQL> explain plan for
with a as (select ename,job,deptno from emp where sal>(select avg(sal) from emp))
select * from a union all select * from a;        

Explained.

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------

--------------------------------------------
Plan hash value: 

--------------------------------------------------------------------------------------------------------
| Id  | Operation                  | Name                      | Rows  | Bytes | Cost (%CPU)  | Time     |
--------------------------------------------------------------------------------------------------------
|    | SELECT STATEMENT           |                           |      |     |         ()| :: |
|    |  TEMP TABLE TRANSFORMATION |                           |       |       |              |          |
|    |   LOAD AS SELECT           | SYS_TEMP_0FD9D6601_4DC46A |       |       |              |          |
|*   |    TABLE ACCESS FULL       | EMP                       |      |     |          ()| :: |
|    |     SORT AGGREGATE         |                           |      |     |              |          |
|    |      TABLE ACCESS FULL     | EMP                       |     |    |          ()| :: |
|    |   UNION-ALL                |                           |       |       |              |          |
|    |    VIEW                    |                           |      |     |          ()| :: |
|    |     TABLE ACCESS FULL      | SYS_TEMP_0FD9D6601_4DC46A |      |     |          ()| :: |
|    |    VIEW                    |                           |      |     |          ()| :: |
|   |     TABLE ACCESS FULL      | SYS_TEMP_0FD9D6601_4DC46A |      |     |          ()| :: |
--------------------------------------------------------------------------------------------------------

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

    - filter("SAL"> (SELECT AVG("SAL") FROM "EMP" "EMP"))

Note
-----
   - dynamic sampling used for this statement (level=)

 rows selected.

充分证明 :
.当with as 语句没有被调用2次以上时,如果表需要访问多次,那么需要加hints  /*+ materialize */ 

.如果with as 语句被调用2次以上时,自动会将 with as 短语的数据放入一个临时表，这个时候不用写materialize hint

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作者：zhaoyangjian724
来源：CSDN
原文：https://blog.csdn.net/zhaoyangjian724/article/details/17692199
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