oracle优化sql的内部过程分析

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Oracle对sql的优化过程如下

查询重写

独立于优化器 ，分两部分

1 子查询展开

分为相关子查询和非相关子查询，将其转化为等价join；
Hint：unnest/no_unnest/hash_sj/hash_aj
通常子查询在最后执行，通过Push_subq可尽早执行http://blog.itpub.net/15480802/viewspace-688364/
参考案例
http://blog.itpub.net/15480802/viewspace-703260/
http://blog.itpub.net/15480802/viewspace-688361/

2 视图合并

或将视图展开，或把外部条件推入视图；不能合并的视图，执行计划会显示View关键字；
Hint: merge/no_merge
限制条件
1 集合操作 union/intersect/minus/union all
2 connect by
3 rownum

查询优化

1 In-list/OR

优化器有3种处理方法
1 IN-list迭代器：
将row source每一行同IN-list值逐一比较，列必须有索引；10157事件可禁用此功能；

2 IN-list扩展：
将IN-list或OR扩展成UNION ALL；CBO必须对每个扩展子句评估cost，且执行时每个分支都要读一次表，可使用NO_EXPAND禁用(与USE_CONCAT相反)，或将IN-list值存入lookup表并join改进；

3 Filter过滤：
采用filter对取出的结果集进行过滤；

案例

DB:11203
create table temp as select object_id,object_name,status,owner from dba_objects;
exec dbms_stats.gather_table_stats('SYS','TEMP');

select object_name,status from temp where object_id in (1,2,3);

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

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1896031711

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     3 |   108 |   188   (2)| 00:00:03 |
|*  1 |  TABLE ACCESS FULL| TEMP |     3 |   108 |   188   (2)| 00:00:03 |
--------------------------------------------------------------------------

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

   1 - filter("OBJECT_ID"=1 OR "OBJECT_ID"=2 OR "OBJECT_ID"=3)

13 rows selected.

2 OR展开

use_concat在11203已经不好使了，需要改为USE_CONCAT( OR_PREDICATES(1) )
select /*+ use_concat */ object_name,status from temp where object_id in (1,2,3)； --依旧使用filter
select /*+ USE_CONCAT( OR_PREDICATES(1) ) */ object_name,status from temp where object_id in (1,2,3)--与下面的union all等价
select object_name,status from temp where object_id =1
union all
select object_name,status from temp where object_id =2
union all
select object_name,status from temp where object_id =3;

--使用USE_CONCAT(OR_PREDICATES(1))
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     3 |   108 |   562   (1)| 00:00:07 |
|   1 |  CONCATENATION     |      |       |       |            |          |
|*  2 |   TABLE ACCESS FULL| TEMP |     1 |    36 |   187   (1)| 00:00:03 |
|*  3 |   TABLE ACCESS FULL| TEMP |     1 |    36 |   187   (1)| 00:00:03 |
|*  4 |   TABLE ACCESS FULL| TEMP |     1 |    36 |   187   (1)| 00:00:03 |
---------------------------------------------------------------------------

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

   2 - filter("OBJECT_ID"=1)
   3 - filter("OBJECT_ID"=2)
   4 - filter("OBJECT_ID"=3)

--直接使用union all
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     3 |   108 |   562  (67)| 00:00:07 |
|   1 |  UNION-ALL         |      |       |       |            |          |
|*  2 |   TABLE ACCESS FULL| TEMP |     1 |    36 |   187   (1)| 00:00:03 |
|*  3 |   TABLE ACCESS FULL| TEMP |     1 |    36 |   187   (1)| 00:00:03 |
|*  4 |   TABLE ACCESS FULL| TEMP |     1 |    36 |   187   (1)| 00:00:03 |
---------------------------------------------------------------------------

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

   2 - filter("OBJECT_ID"=1)
   3 - filter("OBJECT_ID"=2)
   4 - filter("OBJECT_ID"=3)

3 IN-list遍历
要先创建索引
create index ind1 on temp(object_id);

-------------------------------------------------------------------------------------
| Id  | Operation                    | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |      |     3 |   108 |     5   (0)| 00:00:01 |
|   1 |  INLIST ITERATOR             |      |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| TEMP |     3 |   108 |     5   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | IND1 |     3 |       |     4   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

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

   3 - access("OBJECT_ID"=1 OR "OBJECT_ID"=2 OR "OBJECT_ID"=3)

2 星型转换

适用于事实表很小，维度表很大且缺失连接条件

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