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 forwith a as (select /*+ materialize */ ename,job,deptno from emp where sal>(select avg(sal) from emp))select * from a ;  2    3Explained.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 forwith a as (select ename,job,deptno from emp where sal>(select avg(sal) from emp))select * from a ;  2    3Explained.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 forwith 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,如果表需要访问多次,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