现象：一条SQL的运行突然变得很慢。

select uidTable.column_value, first_name||' '||last_name,
company, job_title, upper(member_level), upper(service_value)
from (select * from table(select cast(multiset
(select b from bbb)as Taaa) from dual)) uidTable,member
where uidTable.column_value = member.login_id(+) and
member.site='alibaba' and member.site='test';
 

原因：经检查错误得原因是用户增加了一个条件member.site=test,造成连接的顺序变化了，原来的驱动表是uidTable(最多1024条记录），现在变成了member表做驱动(600W条）。所以这条语句变得特别慢。

但是既然是外连接，为什么连接的顺序会改变呢？因为外连接的连接顺序不是由COST决定的，而是由连接的条件决定的。发现执行计划如下：

-------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
------------------------------------------------
| 0 | SELECT STATEMENT | | 1018 | 72278 | 8155 |
| 1 | NESTED LOOPS | | 1018 | 72278 | 8155 |
| 2 | VIEW | | 4072 | 69224 | 11 |
| 3 | COLLECTION ITERATOR SUBQUERY FETCH| | | | |
| 4 | TABLE ACCESS FULL | DUAL | 4072 | | 11 |
| 5 | TABLE ACCESS FULL | BBB | 41 | 287 | 2 |
| 6 | TABLE ACCESS BY INDEX ROWID | MEMBER | 1 | 54 | 2 |
|* 7 | INDEX UNIQUE SCAN | MEMBER_SITE_LID_PK | 4 | | 1 |
-------------------------------------------------
 

大家要问：“为什么根本就没有执行外连接呢？”问题出在member.site='test'这个条件上，因为对外连接的表加了条件，造成外连接失效。改为member.site(+)='test'后，问题彻底解决。

---------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
-----------------------------------------------------
| 0 | SELECT STATEMENT | | 1018 | 72278 | 8155 |
| 1 | NESTED LOOPS | | 1018 | 72278 | 8155 |
| 2 | VIEW | | 4072 | 69224 | 11 |
| 3 | COLLECTION ITERATOR SUBQUERY FETCH| | | | |
| 4 | TABLE ACCESS FULL | DUAL | 4072 | | 11 |
| 5 | TABLE ACCESS FULL | BBB | 41 | 287 | 2 |
| 6 | TABLE ACCESS BY INDEX ROWID | MEMBER | 1 | 54 | 2 |
|* 7 | INDEX UNIQUE SCAN | MEMBER_SITE_LID_PK | 4 | | 1 |
-----------------------------------------------------