用AUTOTRACE查看执行的计划的同学常问到执行计划里的BUFFER SORT是什么意思,这里为什么要排序呢?
BUFFER SORT不是一种排序,而是一种临时表的创建方式。
BUFFER是执行计划想要表达的重点,是其操作: 在内存中存放一张临时表。
SORT修饰BUFFER,表示具体在内存的什么地方存放临时表: 在PGA的SQL工作区里的排序区。
至少有一种方法可以说服对此表示怀疑的人们,就是查询V$SQL_PLAN_STATISTICS_ALL.PROJECTION字段。
将STATISTICS_LEVEL设置为ALL先,然后执行真-排序命令,比如:select hire_date,salary from hr.employees order by hire_date
然后查看其V$SQL_PLAN_STATISTICS_ALL.PROJECTION字段:
SYS@br//scripts> select projection from v$sql_plan_statistics_all where sql_id=(select sql_id from v$sql where sql_text='select hire_date,salary from hr.employees order by hire_date') and operation='SORT' and options='ORDER BY';

PROJECTION
--------------------------------------------------------------------------------------------------------------------------------------------
(#keys=1) "HIRE_DATE"[DATE,7], "SALARY"[NUMBER,22]
1 row selected.
其中开头的#keys表示返回的结果中排序的字段数量。
再执行一句真-排序命令:select hire_date,salary from hr.employees order by salary,hire_date

然后查看其V$SQL_PLAN_STATISTICS_ALL.PROJECTION字段,#keys因该为２:

SYS@br//scripts> select projection from v$sql_plan_statistics_all where sql_id=(select sql_id from v$sql where sql_text='select hire_date,salary from hr.employees order by salary,hire_date') and operation='SORT' and options='ORDER BY';

PROJECTION
--------------------------------------------------------------------------------------------------------------------------------------------
(#keys=2) "SALARY"[NUMBER,22], "HIRE_DATE"[DATE,7]

1 row selected.

看,这回２了吧,北方的同学不要笑,请忍住。
来看看我们萌萌的BUFFER SORT的表现吧~
执行下面这个查询,它使用了所谓的BUFFER SORT:

select ch.channel_class,c.cust_city,sum(s.amount_sold) sales_amount
from sh.sales s,sh.customers c,sh.channels ch
where s.cust_id=c.cust_id and s.channel_id=ch.channel_id and
c.cust_state_province='CA' and
ch.channel_desc='Internet'
group by ch.channel_class,c.cust_city

附上其执行计划,Id为5的Operation是BUFFER SORT:
execution Plan
----------------------------------------------------------
Plan hash value: 3047021169
----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 133 | 7980 | 902 (2)| 00:00:11 | | |
| 1 | HASH GROUP BY | | 133 | 7980 | 902 (2)| 00:00:11 | | |
|* 2 | HASH JOIN | | 12456 | 729K| 901 (2)| 00:00:11 | | |
| 3 | MERGE JOIN CARTESIAN| | 383 | 18001 | 408 (1)| 00:00:05 | | |
|* 4 | TABLE ACCESS FULL | CHANNELS | 1 | 21 | 3 (0)| 00:00:01 | | |
| 5 | BUFFER SORT | | 383 | 9958 | 405 (1)| 00:00:05 | | |
|* 6 | TABLE ACCESS FULL | CUSTOMERS | 383 | 9958 | 405 (1)| 00:00:05 | | |
| 7 | PARTITION RANGE ALL | | 918K| 11M| 489 (2)| 00:00:06 | 1 | 28 |
| 8 | TABLE ACCESS FULL | SALES | 918K| 11M| 489 (2)| 00:00:06 | 1 | 28 |
----------------------------------------------------------------------------------------------------

查看其V$SQL_PLAN_STATISTICS_ALL.PROJECTION字段:
SYS@br//scripts> select distinct projection from v$sql_plan_statistics_all where sql_id in (select distinct sql_id from v$sql where sql_text like '%where s.cust_id=c.cust_id and s.channel_id=ch.channel_id and%') and operation='BUFFER' and options='SORT';

PROJECTION
--------------------------------------------------------------------------------------------------------------------------------------------
(#keys=0) "C"."CUST_ID"[NUMBER,22], "C"."CUST_CITY"[VARCHAR2,30]
1 row selected.

结果#keys等于0,是0啊... 0意味着该操作根据0个字段排序,那就是没有排序咯。
同样显示SORT但是不SORT打着左灯向右转的还有著名的SORT AGGREGATE。
只能这样说,执行计划操作的取名有时真的太淘气了。

至关心的我的朋友们: 近来更新甚少是因为忙别的事情去了,忙得调不动DBWR了,但是不代表LGWR不工作、Dirty Buffer不产生了,相反它们忙得很、多得不得了。

欢迎访问http://blogs.oracle.com/toddbao
Todd Bao 包光磊[@more@]

ITPub博客

BUFFER SORT是BUFFER却不是SORT

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