使用stored_outline优化、稳定执行计划

SqlTuning 2010-01-04 20:53:07 阅读176 评论0   字号：大中小 订阅

1、打开一个session，设置自动创建sql的stored_outline

    alter session set create_stored_outlines = demo;

2、抓取性能不好的sql，例如（假设nest loop 比较优）：

SELECT /*+ use_hash(a,b)*/* FROM game_draw_stats_site_sale a ,game_draw b  WHERE a.draw_id =b.draw_id AND a.site_id =242023;

     在本session中运行。抓取stored_outline，视图user_outlines，  

     User_Outline_Hints中可查到：

     SELECT * FROM user_outlines;
     SELECT * FROM User_Outline_Hints;

1 SYS_OUTLINE_10010420202142101   DEMO      USED          2010-1-4  20:20:21 10.2.0.2.0 
2 NEW_PLAN                                            DEMO      UNUSED      2010-1-4 20:56:22  10.2.0.2.0 

3、停止收集执行的路径（否则以下你执行的一些SQL也会放到存储概要的表格中，令接下来的处理有点困难）。

　alter session set create_stored_outlines = false；

4、在有问题的sql上创建新的outline

    此时的sql是 加上正确hint的语句，例如本例中将/*+ use_hash(a,b)*/ 换成了 /*+ use_nl(a,b)*/ )  

   create or replace outline new_plan for category demo on SELECT /*+ use_nl(a,b)*/ * FROM  

   game_draw_stats_site_sale a ,game_draw b  WHERE a.draw_id =b.draw_id AND a.site_id =242023;

   其实就是将性能不好的sql的hint名改成现在的这个性能好的hint名，达到优化、固定执行计划的目的。

5、用新创建的outline进行交换：

      update outln.ol$hints
set ol_name =
decode(
ol_name,
'NEW_PLAN','SYS_OUTLINE_10010420202142101',
'SYS_OUTLINE_10010420202142101','NEW_PLAN'
)
where ol_name in ('SYS_OUTLINE_10010420202142101','NEW_PLAN')
;

update outln.ol$ ol1
set hintcount = (
select hintcount
from ol$ ol2
where ol2.ol_name in ('SYS_OUTLINE_10010420202142101','NEW_PLAN')
and ol2.ol_name != ol1.ol_name
)
where
ol1.ol_name in ('SYS_OUTLINE_10010420202142101','NEW_PLAN')
;  

6、启动一个新的session进行测试：

      alter session set use_stored_outlines =demo;

7、测试无误后，修改系统参数：

      alter system set use_stored_outlines = demo;

8、对绑定变量的情况也可参考

SQL> alter system set use_stored_outlines =true;
 
System altered
 
SQL> create or replace outline ol_1 on SELECT * FROM game_draw_stats_site_sale a ,game_draw b  WHERE a.draw_id =b.draw_id AND a.site_id =:1;
 
Outline created
 
SQL> create or replace outline ol_2 on SELECT /*+ use_hash */ * FROM game_draw_stats_site_sale a ,game_draw b  WHERE a.draw_id =b.draw_id AND a.site_id =:1;
 
Outline created
 
SQL> show user
User is "SYS"
 
SQL> UPDATE OUTLN.OL$HINTS SET OL_NAME=DECODE(OL_NAME,'OL_1','OL_2', 'OL_2','OL_1') WHERE OL_NAME IN ('OL_1','OL_2');
 
16 rows updated
 
SQL> DROP OUTLINE OL_2;
 
Outline dropped