刚看了《oracle 高效设计》的讲到的,说到oracle中有标量子查询,性能要由于关联查询,但是我在SQL server中知道关联查询的性能要优于标量子查询。
我们来做个测试,看看性能:执行语句:
set autotrace on
select a.username,count(*) from all_users a,all_objects b
where a.username=b.owner(+)
group by a.username;
select a.username,count(*) from all_users a,all_objects b
where a.username=b.owner(+)
group by a.username;
在SQL developer中执行,结果如下:
5356 recursive calls
0 db block gets
82152 consistent gets
0 db block gets
82152 consistent gets
再执下列语句:
set autotrace on
select a.username,(select count(*) from all_objects b where b.owner=a.username) cnt
from all_users a;
select a.username,(select count(*) from all_objects b where b.owner=a.username) cnt
from all_users a;
执行结果,如下:
5371 recursive calls
0 db block gets
98645 consistent gets
0 db block gets
98645 consistent gets
这时发现,通过执行关联查询的性能要优于标量子查询。估计《Oracle 高效设计》是版本比较低,现在都是用的10gR2的版本有很大改变。
执行语句如下:
set autotrace on
select a.username,count(*),avg(object_id) from all_users a,all_objects b
where a.username=b.owner(+) group by a.username;
select a.username,count(*),avg(object_id) from all_users a,all_objects b
where a.username=b.owner(+) group by a.username;
执行结果:
5371 recursive calls
0 db block gets
82157 consistent gets
0 db block gets
82157 consistent gets
执行语句:
set autotrace on
select username,to_number(substr(data,1,10)) cnt,to_number(substr(data,11)) avg from
(
select a.username,(select to_char(count(*),'fm0000000009') || avg(object_id) from all_objects b where b.owner=a.username) data from all_users a)
select username,to_number(substr(data,1,10)) cnt,to_number(substr(data,11)) avg from
(
select a.username,(select to_char(count(*),'fm0000000009') || avg(object_id) from all_objects b where b.owner=a.username) data from all_users a)
执行结果:
5356 recursive calls
0 db block gets
98556 consistent gets
0 db block gets
98556 consistent gets
执行语句:
set autotrace on
create or replace type myType as object
(cnt number,avg number);
select username,a.data.cnt,a.data.avg from
(select username,(select myType(count(*),avg(object_id)) from all_objects b where b.owner=a.username) data from all_users a) a;
create or replace type myType as object
(cnt number,avg number);
select username,a.data.cnt,a.data.avg from
(select username,(select myType(count(*),avg(object_id)) from all_objects b where b.owner=a.username) data from all_users a) a;
执行结果:
5390 recursive calls
0 db block gets
98662 consistent gets
0 db block gets
98662 consistent gets
总结:
通过上面的测试发现,TOM的《Oracle高效设计》里谈到的标量子查询的性能要由于关联查询,在旧版本的中可能是可以的,在10gR2中是不成立的。