疑问？ Oracle普通的B树索引是升序还是降序？

Posted 2020-12-14

疑问？ Oracle普通的B树索引是升序还是降序？

一、测试创建对象

SQL> create table a as select * from dba_objects;

Table created.

SQL> create index a_id on a(object_id);

Index created.

SQL> set autotrace on 

二、测试索引，观察执行计划

SQL> select object_name from a where object_id in(7788,7799) order by object_id desc;

OBJECT_NAME
--------------------------------------------------------------------------------------------------------------------------------
V$DIAG_DDE_USR_INC_ACT_MAP
V_$DIAG_DDE_USER_ACTION_DEF


Execution Plan
----------------------------------------------------------
Plan hash value: 2600461980

--------------------------------------------------------------------------------------
| Id  | Operation                     | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |      |    14 |  1106 |     9   (0)| 00:00:01 |
|   1 |  INLIST ITERATOR              |      |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID | A    |    14 |  1106 |     9   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN DESCENDING| A_ID |   299 |       |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("OBJECT_ID"=7788 OR "OBJECT_ID"=7799)

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL> select object_name from a where object_id in(7788,7799) order by object_id asc;

OBJECT_NAME
--------------------------------------------------------------------------------------------------------------------------------
V_$DIAG_DDE_USER_ACTION_DEF
V$DIAG_DDE_USR_INC_ACT_MAP


Execution Plan
----------------------------------------------------------
Plan hash value: 2908601139

-------------------------------------------------------------------------------------
| Id  | Operation                    | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |      |    14 |  1106 |     9   (0)| 00:00:01 |
|   1 |  INLIST ITERATOR             |      |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| A    |    14 |  1106 |     9   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | A_ID |   299 |       |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("OBJECT_ID"=7788 OR "OBJECT_ID"=7799)

Note
-----
   - dynamic sampling used for this statement (level=2)

可以发现，使用ASC时，执行计划无改变

使用DESC 时，执行计划存在

INDEX RANGE SCAN DESCENDING 索引范围降序扫描，因此本次实验证明，当默认排序方式创建索引时，索引数据使用升序排列。

那么问题来了，知道这个东西有啥用呢？
创建复合索引，并且业务SQL存在order by 的方式时！！！注意理解，或者SQL order by desc/asc 则创建复合索引列跟着 desc/asc也没毛病。