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$sql where sql_text='select hire_date,salary from hr.employees order by salary,hire_date') and operation='SORT' and options='ORDER BY'; PROJECTION ------------------------------------------------------------------------------------ (#keys=2) "SALARY"[NUMBER,22], "HIRE_DATE"[DATE,7] 1 row selected.
来看看我们萌萌的BUFFER SORT的表现吧~
执行下面这个查询,它使用了所谓的BUFFER SORT:
select ch.channel_class,c.cust_city,sum(s.amount_sold) sales_amount from sh.sales s,sh.customers c,sh.channels ch where s.cust_id=c.cust_id and s.channel_id=ch.channel_id and c.cust_state_province='CA' and ch.channel_desc='Internet' group by ch.channel_class,c.cust_city 附上其执行计划,Id为5的Operation是BUFFER SORT: execution Plan ---------------------------------------------------------- Plan hash value: 3047021169 ---------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop | ---------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 133 | 7980 | 902 (2)| 00:00:11 | | | | 1 | HASH GROUP BY | | 133 | 7980 | 902 (2)| 00:00:11 | | | |* 2 | HASH JOIN | | 12456 | 729K| 901 (2)| 00:00:11 | | | | 3 | MERGE JOIN CARTESIAN| | 383 | 18001 | 408 (1)| 00:00:05 | | | |* 4 | TABLE ACCESS FULL | CHANNELS | 1 | 21 | 3 (0)| 00:00:01 | | | | 5 | BUFFER SORT | | 383 | 9958 | 405 (1)| 00:00:05 | | | |* 6 | TABLE ACCESS FULL | CUSTOMERS | 383 | 9958 | 405 (1)| 00:00:05 | | | | 7 | PARTITION RANGE ALL | | 918K| 11M| 489 (2)| 00:00:06 | 1 | 28 | | 8 | TABLE ACCESS FULL | SALES | 918K| 11M| 489 (2)| 00:00:06 | 1 | 28 | ---------------------------------------------------------------------------------------------------- 查看其V$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,AUTOTRACE中执行计划操作的取名有时真的太淘气了。
转自包光磊的博客：http://blogs.oracle.com/toddbao/entry/buffer_sort%E6%98%AFbuffer%E5%8D%B4%E4%B8%8D%E6%98%AFsort
