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);
1filter
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.
2OR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)
3IN-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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