开启和关闭绑定变量窥探

绑定变量窥视cbo的一个附属功能,关闭他不会影响是用cbo还是rbo,就算关闭了绑定变量窥视,cbo还是会利用别的统计信息(num_distinct,density等)来评估cost和cardinality,只
 
是无法使用直方图信息而已:
 
 
SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
 FROM SYS.x$ksppi x, SYS.x$ksppcv y
 WHERE x.inst_id = USERENV ('Instance')
 AND y.inst_id = USERENV ('Instance')
 AND x.indx = y.indx
 AND x.ksppinm LIKE '%&par%';  2    3    4    5    6  
Enter value for par: peek_user
old   6:  AND x.ksppinm LIKE '%&par%'
new   6:  AND x.ksppinm LIKE '%peek_user%'
 
NAME                   VALUE DESCRIB
------------------------------ -------------------- ------------------------------
 
_optim_peek_user_binds TRUE enable peeking of user binds
 

此时绑定变量开启:



create table test300 as select * from dba_objects;

SQL> create table test300 as select * from dba_objects;

表已创建。

SQL> create index test300_idx1 on test300(owner);

索引已创建。


SQL> set linesize 200
SQL> /

OWNER											     COUNT(*)
---------------------------