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首先说下什么叫做内联视图
select * from () view_a v , table_name t where v.id = t.id
就是说from后面有视图存在。
在这样的情况下优化器对安装CBO来决定是不是要把这个视图给拆开，也就是是否合并视图。我们判断内联视图有没有合并，可以通过执行计划来查看，如果执行计划中看到VIEW的关键字，说明视图没有合并，视图是作为一个整体与其他部分连接，如果没看到VIEW，说明视图被优化器合并了。我们来看看下面的例子
举例：SQL> conn scott/tigerConnected.SQL>SQL> create table t_obj as select * from dba_objects;Table created.SQL> create table t_user as select * from dba_users;Table created.SQL> create table t_segments as select * from dba_segments;Table created.--建立一个视图create view v_test asselect o.*,u.ACCOUNT_STATUS,u.LOCK_DATE,u.EXPIRY_DATEfromt_obj oinner joint_user uon o.OWNER= u.USERNAME;--查看内联视图sql的执行计划SQL> explain plan forselect * fromv_test vinner joint_segments son v.OWNER=s.OWNER; 2 3 4 5 6Explained.SQL> set linesize 400SQL> SELECT * FROM TABLE(dbms_xplan.display);PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Plan hash value: 1435736869----------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |----------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 33M| 18G| 434 (25)| 00:00:06 ||* 1 | HASH JOIN | | 33M| 18G| 434 (25)| 00:00:06 || 2 | TABLE ACCESS FULL | T_SEGMENTS | 5913 | 1882K| 34 (0)| 00:00:01 ||* 3 | HASH JOIN | | 78247 | 19M| 296 (1)| 00:00:04 || 4 | TABLE ACCESS FULL| T_USER | 35 | 1855 | 3 (0)| 00:00:01 || 5 | TABLE ACCESS FULL| T_OBJ | 78247 | 15M| 292 (1)| 00:00:04 |PLAN_TABLE_OUTPUT----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------1 - access("O"."OWNER"="S"."OWNER")3 - access("O"."OWNER"="U"."USERNAME")Note------ dynamic sampling used for this statement (level=2)
上面的执行计划我们看不到VIEW关键字，但是原SQL是是有VIEW的，说明发生了视图合并的操作
SQL> explain plan forselect /*+ no_merge(v)*/* fromv_test vinner joint_segments son v.OWNER=s.OWNER; 2 3 4 5 6Explained.SQL> SELECT * FROM TABLE(dbms_xplan.display);PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Plan hash value: 2926968361-----------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |-----------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 33M| 17G| 434 (25)| 00:00:06 ||* 1 | HASH JOIN | | 33M| 17G| 434 (25)| 00:00:06 || 2 | TABLE ACCESS FULL | T_SEGMENTS | 5913 | 1882K| 34 (0)| 00:00:01 || 3 | VIEW | V_TEST | 78247 | 18M| 296 (1)| 00:00:04 ||* 4 | HASH JOIN | | 78247 | 19M| 296 (1)| 00:00:04 || 5 | TABLE ACCESS FULL| T_USER | 35 | 1855 | 3 (0)| 00:00:01 |PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| 6 | TABLE ACCESS FULL| T_OBJ | 78247 | 15M| 292 (1)| 00:00:04 |-----------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------1 - access("V"."OWNER"="S"."OWNER")4 - access("O"."OWNER"="U"."USERNAME")Note-----PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- dynamic sampling used for this statement (level=2)23 rows selected.
在看看上面的执行计划我们在原sql里加了一个hint. /*+ no_merge(v)*/这个就相当于告诉优化器，别自作多情，我的视图不需要合并，也就出现了VIEW的关键字
上面就是视图合并的大概了，但是大家看到这里肯定会说，这些我也知道啊，他合并不合并有什么区别，和我有什么关系，那么请看下面的总结
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视图合并对打乱原来视图里面表的连接顺序，但是有时候我们是不想的，我们就是想视图先执行，然后生成一个结果集与其他表连接
create view v_test asselect o.*,u.ACCOUNT_STATUS,u.LOCK_DATE,u.EXPIRY_DATEfromt_obj oinner joint_user uon o.OWNER= u.USERNAME;select * fromv_test vinner joint_segments son v.OWNER=s.OWNER;也就是对于上面的sql, 最后d 执行顺序可能是t_obj 先和t_segments 连接，然后在和t_user连接， 这样有时候，会出现问题，可能会产生笛卡尔积哦，死翘翘！~
2. 对于如下形式的sql
select ....... from () t1 ,() t2 where t1.id=t2.id
这样的sql跑起来很慢，但是但是执行t1里面的select很快，单独执行t2里面的select也很快，但是放一起就死慢，那就要注意是不是视图合并坑了你，我们是不是要手动加hint /*+ no_merge(t1) */或者 hint /*+ use_hash(t1,t2) */, 多个内联视图推荐后面的方法。然后再去试试性能吧，也行是你意向不到的。
3. 在大数量情况下，数据库的优化器还是很智能的，CBO来决定合并或者不合并，但是有时候发现我们有视图就是不能合并，怎么办呢？
那就看看你的视图是不是含有下面一些关键字：
union, union all, start with connect by, rownum, cube
如果视图里面有上面的东东，抱歉，视图只能作为一个整体，不能合并，不要错怪优化器哦。下面我们来看一个例子。
所以这里强调一点，上面的东东，写sql的时候要注意哦，可能不经意间就坑了你。
新建一个带有rownum的视图，没建hint /*+ no_merge(v) */哦，但是我们还是能看到VIEW，说明视图没有合并
SQL> create view v_test_rownum asselect o.*,u.ACCOUNT_STATUS,u.LOCK_DATE,u.EXPIRY_DATEfromt_obj oinner joint_user uon o.OWNER= u.USERNAMEwhere rownum < =1000; 2 3 4 5 6 7 8View created.SQL> explain plan forselect * fromv_test_rownum vinner joint_segments son v.OWNER=s.OWNER; 2 3 4 5 6Explained.SQL> SELECT * FROM TABLE(dbms_xplan.display);PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Plan hash value: 4218700277---------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |---------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 422K| 229M| 46 (7)| 00:00:01 ||* 1 | HASH JOIN | | 422K| 229M| 46 (7)| 00:00:01 || 2 | VIEW | V_TEST_ROWNUM | 1000 | 237K| 10 (10)| 00:00:01 ||* 3 | COUNT STOPKEY | | | | | ||* 4 | HASH JOIN | | 78247 | 34M| 10 (10)| 00:00:01 || 5 | TABLE ACCESS FULL| T_USER | 35 | 1855 | 3 (0)| 00:00:01 |PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| 6 | TABLE ACCESS FULL| T_OBJ | 78247 | 15M| 7 (15)| 00:00:01 || 7 | TABLE ACCESS FULL | T_SEGMENTS | 5913 | 1882K| 34 (0)| 00:00:01 |---------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------1 - access("V"."OWNER"="S"."OWNER")3 - filter(ROWNUM<=1000)4 - access("O"."OWNER"="U"."USERNAME")PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Note------ dynamic sampling used for this statement (level=2)
今天说到这里。
个人理解，错误望指正，谢谢