参考资料：
本系列博客主要参考资料有CUUG冉乃纲老师数据库教学笔记，《SQL优化核心思想》（罗炳森，黄超，钟侥著），《PostgreSQL技术内幕：查询优化深度探索》（张树杰著），排名不分先后。
9.1 提取公共项
9.1.1. 数据脚本
drop table test1;
drop table test2;
create table test1 as select * from dba_objects;
create table test2 as select * from dba_objects;
insert into test2 select * from test2;
--反复执行test2的自插操作，直到行数足够多
commit;
drop index ix_test1;
drop index ix_test2;
create index ix_test1_01 on test1(owner);
create index ix_test1_02 on test1(object_id);
9.1.2 改写过程
（1）改写前
select '1' as seq, count(t1.OBJECT_ID),count(t2.OBJECT_ID)
from test1 t1
left join (select * from test2 t where t.OBJECT_ID<1000 and t.OWNER='OUTLN') t2
on t1.OBJECT_ID=t2.OBJECT_ID
where t1.OBJECT_ID<1000
union
select '2', count(t1.OBJECT_ID),count(t2.OBJECT_ID)
from test1 t1
left join (select * from test2 t where t.OBJECT_ID<1000 and t.OBJECT_TYPE='EDITION') t2
on t1.OBJECT_ID=t2.OBJECT_ID
where t1.OBJECT_ID<1000;
改写前物理读和逻辑多都很大
（2）第一次改写，提取公共项目，减少大表test2的扫描次数
with tmp as
(select t.*
from test2 t
where t.OBJECT_ID<1000
and rownum>0)
select '1' as seq,count(t1.OBJECT_ID),count(t2.OBJECT_ID)
from test1 t1
left join tmp t2
on t1.OBJECT_ID=t2.OBJECT_ID
and t2.OWNER='OUTLN'
where t1.OBJECT_ID<1000
union
select '2', count(t1.OBJECT_ID),count(t2.OBJECT_ID)
from test1 t1
left join tmp t2
on t1.OBJECT_ID=t2.OBJECT_ID
and t2.OBJECT_TYPE='EDITION'
where t1.OBJECT_ID<1000;
通过下面执行计划，可见提取公共项目后，和预期相反，执行时间反倒变长了。
原因是出现了1515个物理写，因为数据量较大，需要将with的数据写到磁盘上，让时间增加。
（3）改写方案就是把test2表的检索条件都加大with里，减少行数，同时，只是检索需要的项目，同时从两个方向减少内存占用。
with tmp as
(select t.OWNER, t.OBJECT_ID,t.OBJECT_TYPE
from test2 t
where t.OBJECT_ID<1000
and (t.OWNER='OUTLN' or t.OBJECT_TYPE='EDITION'))
select '1' as seq, count(t1.OBJECT_ID),count(t2.OBJECT_ID)
from test1 t1
left join tmp t2
on t1.OBJECT_ID=t2.OBJECT_ID
and t2.OWNER='OUTLN'
where t1.OBJECT_ID<1000
union
select '2', count(t1.OBJECT_ID),count(t2.OBJECT_ID)
from test1 t1
left join tmp t2
on t1.OBJECT_ID=t2.OBJECT_ID
and t2.OBJECT_TYPE='EDITION'
where t1.OBJECT_ID<1000;
通过执行计划，逻辑读，物理读都大幅度降低，虽然还有4个物理写，还是把性能提高了接近一倍。
9.1.3 结论
（1）一个大表被引用多次，IO成本较大时，可以用提取公共项的方式提升SQL性能；
（2）注意点：With提取时，从检索条件（选择）和检索项目（投影）尽量减少with结果集的大小，以避免With结果集太大，产生物理写等其他性能问题。
9.2 With改善Filter
9.2.1 数据脚本
drop table test1;
drop table test2;
create table test1 as select * from dba_objects;
create table test2 as select * from dba_objects;
insert into test2 select * from test2;
insert into test2 select * from test2;
commit;
create index ix_test1_01 on test1(object_id);
create index ix_test2_01 on test2(object_id);
9.2.2 改写过程
（1）改写前
EXPLAIN PLAN FOR
select count(*)
from test1 t1
where exists (select 1
from test2 t2
where t1.owner=t2.owner
and t1.object_type = t2.object_type
and t1.OBJECT_NAME=t2.OBJECT_NAME
group by t2.owner,t2.object_type,OBJECT_NAME
having count(*)>30)
and t1.object_id>70000;
test2是一张大表，执行计划走了Filter连接，意味着主查询返回多少条，子查询的大表test2就要全表扫描多少次，IO成本直接把性能卡死了。
这里有两点需要说明：
①改善方法至少有两种，第一种是with改写，消除filter，在本节重点讨论，第二种子查询改成表连接，根据t2.owner,t2.object_type,OBJECT_NAME的选择性和最终返回数据量建立索引，让Filter连接走索引，在本系列<SQL优化方法><第二章 Filter连接那点事>进行分析；
②Filter连接并不是一无是处，我们有时候要善待它，不要见一次砍死一次，也在本系列<SQL优化方法><第二章 Filter连接那点事>进行分析。
（2）
with tmp as
(select t2.owner,t2.object_type ,t2.OBJECT_NAME
from test2 t2
where rownum>0
group by t2.owner,t2.object_type,OBJECT_NAME
having count(*)>30)
select count(*)
from test1 t1
where exists (select 1
from tmp t2
where t1.owner=t2.owner
and t1.object_type = t2.object_type
and t1.OBJECT_NAME=t2.OBJECT_NAME)
and t1.object_id>70000;
9.2.3 结论
如果Filter连接需要改善，可以把子查询拿出来做成with，取消Filter连接。让主查询和with走Hash，注意是走Hash，不是嵌套循环，因为同with子句连接无法走索引。
