在表上建复合索引是常见的事情，那索引中键值的顺序是什么呢？通过下面的实验可以得出三个结论：

--制造实验数据，并收集统计信息

SQL> create table test as select * from dba_objects;
SQL> create index ind_id_owner on test(object_id,owner);
SQL> create index ind_owner_id on test(owner,object_id);
SQL> exec dbms_stats.gather_table_stats(user,'test',cascade=>true);

SQL> select count(distinct owner) owner_count,  count(distinct object_id) id_count, count(*)  from test;

OWNER_COUNT   ID_COUNT   COUNT(*)
        -----------            ----------        ----------
         25                     50583      50584
SQL> set autotrace traceonly

1. 如果是等值查询，那字段无先后之分，结合着索引的存储结构，索引存储的是键值和rowid，复合索引就是存储两个键值，如果是等值查询，可以看作是一个值，跟单个字段索引查询一样。

SQL> select /*+index(test,ind_id_owner)*/* from test    where owner ='TEST' and object_id =52623;
执行计划
----------------------
Plan hash value: 1096520809
--------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------
|   0 | SELECT STATEMENT            |              |     1 |    93 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST         |     1 |    93 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IND_ID_OWNER |     1 |       |     1   (0)| 00:00:01 |
--------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID"=52623 AND "OWNER"='TEST')
统计信息
----------------------
          1  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
       1199  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> select /*+index(test,ind_owner_id)*/* from test  where owner ='TEST' and object_id =52623;
执行计划
----------------------
Plan hash value: 724495818
--------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------
|   0 | SELECT STATEMENT            |              |     1 |    93 |     2