·BUFFER SORT是BUFFER却不是SORT
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。
·RIGHT SEMI
HASH JOIN RIGHT SEMI是exisit判式的一个特殊操作。HASH JOIN是Oracle优化两个表（其中一个数据表数据量较小，而另一个数据量较大）连接的一种方式。因为在整个查询中，子查询的外部表 business.accintvoucher a （视作左表）与内部表 business.accmainvoucher （视作右表）实际进行的是连接操作，这不可避免的会产生重复记录。Oracle利用SEMI策略来防 止这一情况，即针对 外部表business.accmainvoucher a的中每一行，一旦与内部表business.accmainvoucher 中的某条记录 匹配成功，将立即返回。
Oracle官方文档对SEMI这种策略进行了定义。
A semi-join returns rows that match an EXISTS subquery without duplicating rows from the left side of the predicate when multiple rows on the right side satisfy the criteria of the subquery。
也就是说，SEMI连接是exists子查询的一种特殊处理方式，semi join最主要的使用场景就是解决exist in。