oracle优化sql的内部过程分析

数据库 2024年05月10日 11:21

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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:11203create 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 filterSQL> 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filterselect /*+ 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 allselect object_name,status from temp where object_id =2union allselect 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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