Oracle压缩索引-如何确定要压缩的列

我想为Oracle应用程序表创建以下索引

create index xxhr_api_transactions_idx1 on hr.hr_api_transactions (status, process_name, nvl(selected_person_id, -1)) compress 3

该表共有62421行。状态列中有10个不同的值。

过程名称

列中有23个不同的值。17419在

selected\u person\u id

列中有不同的值。只有43530个值存在于

selected_person_id

列中，其余值为空（新员工工作流中该人员尚不存在）

我的问题类似于：

select *
from   hr.hr_api_transactions psth   
where  psth.process_name in ('TFG_HR_NEW_HIRE_PLACE_JSP_PRC', 'HR_NEW_HIRE_PLACE_JSP_PRC', 'HR_NEWHIRE_JSP_PRC')   -- TFG specific.
--and    nvl(psth.selected_person_id, -1) in (:p_person_id, -1)   -- 1118634
and    psth.status not in ('W', 'S')   -- Work in Progress, Saved For Later.

---

我的问题是我应该使用压缩3还是压缩2？用62421个总数中的17419个不同值（以及18891个空值）压缩

所选的\u person\u id

列是否更好？

压缩声明和建议是出了名的糟糕。这是你真正需要自己测试的任务之一。您可以通过检查

DBA_SEGMENTS.BYTES

来测试压缩

压缩是CPU和大小之间的权衡。根据我的经验，基本索引压缩的CPU开销非常小。只要大小小于百分之几，我建议使用增加的压缩设置

使用以下代码测试不压缩到

compress 3

的段大小。确保使用足够大的数据进行测试。Oracle在区段中分配空间；如果您使用一个小的测试大小，那么您将只测量数据块大小开销

drop index hr.xxhr_api_transactions_idx1;
create index xxhr_api_transactions_idx1 on hr.hr_api_transactions (status, process_name, nvl(selected_person_id, -1));
select bytes/1024/1024/1024 gb from dba_segments where segment_name = 'XXHR_API_TRANSACTIONS_IDX1';

drop index hr.xxhr_api_transactions_idx1;
create index xxhr_api_transactions_idx1 on hr.hr_api_transactions (status, process_name, nvl(selected_person_id, -1)) compress 1;
select bytes/1024/1024/1024 gb from dba_segments where segment_name = 'XXHR_API_TRANSACTIONS_IDX1';

drop index hr.xxhr_api_transactions_idx1;
create index xxhr_api_transactions_idx1 on hr.hr_api_transactions (status, process_name, nvl(selected_person_id, -1)) compress 2;
select bytes/1024/1024/1024 gb from dba_segments where segment_name = 'XXHR_API_TRANSACTIONS_IDX1';

drop index hr.xxhr_api_transactions_idx1;
create index xxhr_api_transactions_idx1 on hr.hr_api_transactions (status, process_name, nvl(selected_person_id, -1)) compress 3;
select bytes/1024/1024/1024 gb from dba_segments where segment_name = 'XXHR_API_TRANSACTIONS_IDX1';