1 数据准备
drop table test1;
drop table test2;
create table test1 as select * from dba_objects;
alter table test1 add constraint pk_test1 primary key (object_id);
create table test2 as select * from dba_objects;
insert into test2 select * from test2;
insert into test2 select * from test2;
commit;
2 优化分析
2.1 写法一
select t1.object_id,t1.object_name,t1.OWNER,
t1.object_type,t1.namespace,t1.created,
sum(t2.data_object_id) as sum_id
from test1 t1
inner join test2 t2
on t1.object_id=t2.object_id
group by t1.object_id,t1.object_name,t1.OWNER,
t1.object_type,t1.namespace,t1.created;
因为test1的主键是object_id,并且test2表的字段只有聚合运算，也就意味着只要按照test1的object_id进行聚合就好了，其他的聚合字段是没要的，但是，基于SQL的语法规则，必须要加上其他检索字段，造成了不必要的分组运算。
2.2 写法二
with tmp as
(select t1.object_id,
sum(t2.data_object_id) as sum_id
from test1 t1
inner join test2 t2
on t1.object_id=t2.object_id
group by t1.object_id)
select tt.object_id,tt.object_name,tt.OWNER,
tt.object_type,tt.namespace,tt.created,
t.sum_id
from tmp t
,test1 tt
where t.object_id=tt.OBJECT_ID;
这一次改写，聚合运算明显减少，但是，执行计划走了嵌套循环，不适合当前的检索，我们下面用hint稍加调整。
2.3 写法三
with tmp as
(select /*+ use_hash(t1 t2)*/ t1.object_id,
sum(t2.data_object_id) as sum_id
from test1 t1
inner join test2 t2
on t1.object_id=t2.object_id
group by t1.object_id)
select tt.object_id,tt.object_name,tt.OWNER,
tt.object_type,tt.namespace,tt.created,
t.sum_id
from tmp t
,test1 tt
where t.object_id=tt.OBJECT_ID;
hint调整之后，性能还是没有提升，原因是test1表要多扫描一次，IO成本增加。
2.4 写法四
select t1.object_id, t1.object_name,t1.OWNER,
t1.object_type,t1.namespace,t1.created,
t2.sum_id
from test1 t1
inner join (select t.object_id,sum(t.data_object_id) as sum_id
from test2 t
group by t.object_id) t2
on t1.object_id=t2.object_id;
这一次改写，先聚合，再连接，时间大幅减少。
