现象：一条SQL突然运行的特别慢。
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,memberwhere uidTable.column_value = member.login_id(+) and member.site='alibaba' and member.site='test'; |
出错原因：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）。所以这条语句变的巨慢。
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 |------------------------------------------------- |
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 |----------------------------------------------------------- |
