WITH AS and materialize hints

FROM http://blog.csdn.net/zhaoyangjian724/article/details/17692199?ADUIN=604178834&ADSESSION=1388405668&ADTAG=CLIENT.QQ.5281_.0&ADPUBNO=26283

 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 ;
   2    3
 Explained.

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

 PLAN_TABLE_OUTPUT
 -------------------------------------------------------------------------------------------------------
 Plan hash value: 2006423466

 -------------------------------------------------------------------------------------------------------
 | Id  | Operation                  | Name                     | Rows  | Bytes | Cost (%CPU)| Time     |
 -------------------------------------------------------------------------------------------------------
 |   0 | SELECT STATEMENT           |                          |     1 |    26 |     8   (0)| 00:00:01 |
 |   1 |  TEMP TABLE TRANSFORMATION |                          |       |       |            |          |
 |   2 |   LOAD AS SELECT           | SYS_TEMP_0FD9D6605_E16CE |       |       |            |          |
 |*  3 |    TABLE ACCESS FULL       | EMP                      |     1 |    21 |     3   (0)| 00:00:01 |
 |   4 |     SORT AGGREGATE         |                          |     1 |     4 |            |          |
 |   5 |      TABLE ACCESS FULL     | EMP                      |    14 |    56 |     3   (0)| 00:00:01 |
 |   6 |   VIEW                     |                          |     1 |    26 |     2   (0)| 00:00:01 |
 |   7 |    TABLE ACCESS FULL       | SYS_TEMP_0FD9D6605_E16CE |     1 |    17 |     2   (0)| 00:00:01 |
 -------------------------------------------------------------------------------------------------------

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

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

 19 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 ;  2    3

 Explained.

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

 PLAN_TABLE_OUTPUT
 ----------------------------------------------------------------------------
 Plan hash value: 1876299339

 ----------------------------------------------------------------------------
 | Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
 ----------------------------------------------------------------------------
 |   0 | SELECT STATEMENT    |      |     1 |    21 |     6   (0)| 00:00:01 |
 |*  1 |  TABLE ACCESS FULL  | EMP  |     1 |    21 |     3   (0)| 00:00:01 |
 |   2 |   SORT AGGREGATE    |      |     1 |     4 |            |          |
 |   3 |    TABLE ACCESS FULL| EMP  |    14 |    56 |     3   (0)| 00:00:01 |
 ----------------------------------------------------------------------------

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

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

 15 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;  2    3

 Explained.

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

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

 --------------------------------------------
 Plan hash value: 2575088720

 --------------------------------------------------------------------------------------------------------
 | Id  | Operation          | Name              | Rows  | Bytes | Cost (%CPU)| Time     |
 --------------------------------------------------------------------------------------------------------
 |   0 | SELECT STATEMENT       |                   |     2 |    52 |     4  (50)| 00:00:01 |
 |   1 |  TEMP TABLE TRANSFORMATION |                   |       |       |        |          |
 |   2 |   LOAD AS SELECT       | SYS_TEMP_0FD9D6601_4DC46A |       |       |        |          |
 |*  3 |    TABLE ACCESS FULL       | EMP               |     1 |    39 |     3   (0)| 00:00:01 |
 |   4 |     SORT AGGREGATE     |                   |     1 |    13 |        |          |
 |   5 |      TABLE ACCESS FULL     | EMP               |    14 |   182 |     3   (0)| 00:00:01 |
 |   6 |   UNION-ALL        |                   |       |       |        |          |
 |   7 |    VIEW            |                   |     1 |    26 |     2   (0)| 00:00:01 |
 |   8 |     TABLE ACCESS FULL      | SYS_TEMP_0FD9D6601_4DC46A |     1 |    26 |     2   (0)| 00:00:01 |
 |   9 |    VIEW            |                   |     1 |    26 |     2   (0)| 00:00:01 |
 |  10 |     TABLE ACCESS FULL      | SYS_TEMP_0FD9D6601_4DC46A |     1 |    26 |     2   (0)| 00:00:01 |
 --------------------------------------------------------------------------------------------------------

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

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

 Note
 -----
    - dynamic sampling used for this statement (level=2)

 26 rows selected.

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

 2.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