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

1 - filter("HOTEL_ID"=20)
2 - access("RATE_CATEGORY"=21)

该输出清晰地显示索引 BOOK_IDX2 已使用。为什么现在使用了索引？注意“Rows”列下方的值 (5106)。优化程序正确地确定了值组合的行数的估计值，而非分开的各个值的行数的估计值。

当然了，对于其他的条件，oracle也可以做出准确的判断

SQL> set autotrace trace exp
SQL> select hotel_id,rate_category from books where hotel_id=10 and rate_category=12;

Execution Plan
----------------------------------------------------------
Plan hash value: 2688610195

---------------------------------------------------------------------------
| Id? | Operation? | Name? | Rows? | Bytes | Cost (%CPU)| Time? |
---------------------------------------------------------------------------
|? 0 | SELECT STATEMENT? |? | 39385 |? 230K|? 47? (3)| 00:00:01 |
|*? 1 |? TABLE ACCESS FULL| BOOKS | 39385 |? 230K|? 47? (3)| 00:00:01 |
---------------------------------------------------------------------------

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

1 - filter("RATE_CATEGORY"=12 AND "HOTEL_ID"=10)

在视图 USER_STAT_EXTENSIONS 中，您可以看到在数据库中定义的扩展统计信息：
SQL> select extension_name, extension
2? from user_stat_extensions
3? where table_name='BOOKS';

EXTENSION_NAME? EXTENSION
------------------------------ --------------------------------------------------------------------------------
SYS_STUW3MXAI1XLZHCHDYKJ9E4K90 ("HOTEL_ID","RATE_CATEGORY")