创建一个常规的堆表，并在上面建一个复合索引

SQL> create table t1 as select * from all_objects;

表已创建。

SQL> create index idx_t1_1 on t1(owner,object_type);

索引已创建。

下面这个查询语句比较特殊，
order by的条件与select的列表栏位都在符合索引内，
所以这个查询的检索根本没有涉及到堆表，而完全在索引内完成。
而且因为索引本身就是依序存储的，所以查询计划中并没有sort这一项。

SQL> set autotrace traceonly explain;
SQL> select owner,object_type from t1
2 where owner between 'T' and 'Z'
3 and object_type is not null
4 order by owner,object_type;

执行计划
----------------------------------------------------------
Plan hash value: 2370119825

-----------------------------------------------------------------------------
| Id | Operation        | Name     | Rows | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT |          | 4835 |   132K|    17   (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| IDX_T1_1 | 4835 |   132K|    17   (0)| 00:00:01 |
-----------------------------------------------------------------------------

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

   1 - access("OWNER">='T' AND "OWNER"<='Z')
       filter("OBJECT_TYPE" IS NOT NULL AND "OWNER">='T' AND
              "OWNER"<='Z')

Note
-----
   - dynamic sampling used for this statement

下面的查询也类似，虽然排序条件中指定使用降序，但是db需要做的只是反向扫描索引。
所以仍然没有sort的步骤

SQL> select owner,object_type from t1
2 where owner between 'T' and 'Z'
3 and object_type is not null
4 order by owner desc,object_type desc;

执行计划
----------------------------------------------------------
Plan hash value: 2755489192

----------------------------------------------------------------------------------------
| Id | Operation                   | Name     | Rows | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |          | 4835 |   132K|    17   (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN DESCENDING| IDX_T1_1 | 4835 |   132K|    17   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

   1 - access("OWNER">='T' AND "OWNER"<='Z')
       filter("OBJECT_TYPE" IS NOT NULL AND "OWNER">='T' AND "OWNER"<='Z')

Note
-----
   - dynamic sampling used for this statement

可以看出，下面的查询使查询计划多出了一个sort的步骤。
这是因为，虽然order by和select列表的栏位都在索引内，且数据也只会在索引存储内取得，
但是因为索引都是依升序排列的，所以在取得数据后，db还是要针对第二个栏位进行排序。

SQL> select owner,object_type from t1
2 where owner between 'T' and 'Z'
3 and object_type is not null
4 order by owner,object_type desc;

执行计划
----------------------------------------------------------
Plan hash value: 1804762688

--------------------------------------------------------------------------------------
| Id | Operation         | Name     | Rows | Bytes |TempSpc| Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |          | 4835 |   132K|       |    60   (4)| 00:00:01 |
|   1 | SORT ORDER BY    |          | 4835 |   132K|   360K|    60   (4)| 00:00:01 |
|* 2 |   INDEX RANGE SCAN| IDX_T1_1 | 4835 |   132K|       |    17   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

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

   2 - access("OWNER">='T' AND "OWNER"<='Z')
       filter("OBJECT_TYPE" IS NOT NULL)

Note
-----
   - dynamic sampling used for this statement

接下来创建一个相同的堆表，但是建复合索引时指定第二个栏位为降序

SQL> create table t2 as select * from all_objects;

表已创建。

SQL> create index idx_t2_1 on t2(owner,object_type desc);

索引已创建。

执行以下查询，会发现，因为索引定义时指定第二个栏位为降序，该语句的执行计划没有sort这一步骤。

SQL> select owner,object_type from t2
2 where owner between 'T' and 'Z'
3 and object_type is not null
4 order by owner,object_type desc;

执行计划
----------------------------------------------------------
Plan hash value: 2241212547

-----------------------------------------------------------------------------
| Id | Operation        | Name     | Rows | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT |          | 1004 | 28112 |     5   (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| IDX_T2_1 | 1004 | 28112 |     5   (0)| 00:00:01 |
-----------------------------------------------------------------------------

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

   1 - access("OWNER">='T' AND "OWNER"<='Z')
       filter(SYS_OP_UNDESCEND(SYS_OP_DESCEND("OBJECT_TYPE")) IS NOT
              NULL AND "OWNER">='T' AND "OWNER"<='Z')

Note
-----
   - dynamic sampling used for this statement

但是当order by条件变为全部为升序时，sort的步骤再次出现。

SQL> select owner,object_type from t2
2 where owner between 'T' and 'Z'
3 and object_type is not null
4 order by owner,object_type;

执行计划
----------------------------------------------------------
Plan hash value: 3901850472

------------------------------------------------------------------------------
| Id | Operation         | Name     | Rows | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |          | 1004 | 28112 |     6 (17)| 00:00:01 |
|   1 | SORT ORDER BY    |          | 1004 | 28112 |     6 (17)| 00:00:01 |
|* 2 |   INDEX RANGE SCAN| IDX_T2_1 | 1004 | 28112 |     5   (0)| 00:00:01 |
------------------------------------------------------------------------------

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

   2 - access("OWNER">='T' AND "OWNER"<='Z')
       filter(SYS_OP_UNDESCEND(SYS_OP_DESCEND("OBJECT_TYPE")) IS NOT
              NULL)

Note
-----
   - dynamic sampling used for this statement

SQL> spool off

以上是一个简单的例子。当表变得比较大时，一个排序的任务可能会占用较多的资源和时间。
当遭遇性能问题时，有时可以尝试结合复合索引及排序条件看有没有改善空间。