ORACLE执行计划(7)---采用TRUNC(SYSDATE)-1方法 无法确定分区

转自 http://blog.csdn.net/zengmuansha/article/details/7492826

在看分区的执行计划当中 说明字段有各种不同的分区操作其中有两个

PARTITION RANGE ITERATOR
PARTITION RANGE single

ITERATOR的成本高于SINGLE 好像ITERATOR是跨分区,当没有跨多个分区似的

还有个PARTITION RANGE ITERATOR ALL 这就是访问所有分区.

不管怎么样,不太喜欢PARTITION RANGE ITERATOR

select PROVCODE,AREACODE,to_char(CREATETIME,'yyyymmdd')
from  sms_mms_send_his

where status = 100
and CREATETIME >= trunc(sysdate)-1
and CREATETIME < trunc(sysdate)

SELECT STATEMENT REMOTE, GOAL = ALL_ROWS Depth=0  Operation=SELECT STATEMENT Cost=20295   CPU cost=3824735837 IO cost=20128
 FILTER Depth=1  Operation=FILTER  Filter predicates=TRUNC(SYSDATE@!)-1<TRUNC(SYSDATE@!)   
  PARTITION RANGE ITERATOR Depth=2  Operation=PARTITION RANGE Cost=20295   CPU cost=3824735837 IO cost=20128
   TABLE ACCESS FULL Depth=3  Operation=TABLE ACCESS Cost=20295 Filter predicates="STATUS"=100 AND "CREATETIME">=TRUNC(SYSDATE@!)-1 AND "CREATETIME"<TRUNC(SYSDATE@!)  CPU cost=3824735837 IO cost=20128

如果我指定分区的话

select PROVCODE,AREACODE,to_char(CREATETIME,'yyyymmdd')
from sms_mms_send_his_info  partition(SMS_MMS_SEND_HIS_INFO_201210421)
where status = 100

SELECT STATEMENT, GOAL = ALL_ROWS Depth=0  Operation=SELECT STATEMENT Cost=2   CPU cost=7121 IO cost=2
 PARTITION RANGE SINGLE Depth=1  Operation=PARTITION RANGE Cost=2   CPU cost=7121 IO cost=2
  TABLE ACCESS FULL Depth=2  Operation=TABLE ACCESS Cost=2 Filter predicates="STATUS"=100  CPU cost=7121 IO cost=2

从执行计划上来看 ITERATOR 高于 SINGLE 而且createtime是分区关键字 而它去不是ACCESS谓词,就是定位的谓词条件,反而变成了过滤条件.

为了避免这样情况出现 

and CREATETIME >= trunc(sysdate)-1
and CREATETIME < trunc(sysdate)

改成

and CREATETIME >= to_date('','yyyymmdd')

and CREATETIME < to_date('','yyyymmdd') 

方式

外加/*+full()*/ 提示 避免它走分区本地索引. 虽然我们的DBA说trunc(sysdate)实际执行的时候不跨分区,还是把控制权掌握自己手上好