情况4：
考虑以下情况，已经对一个字符钱建立了索引。这个列只包含数据数据。如果使用以下语法来查询：
select * from t where indexed_colum=5;
注意查询中的数字5是常数5(而不是一个字符串)，此时就没有使用INDEXED_COLUMN上的索引。这是因为，前面的查询等价于以下查询：
select * from t where to_number(indexed_column)=5;
我们对这个列隐式地应用了一个函数，如情况3所述，这就会禁止使用这个索引。
zx@ORCL>create table t2 (x char(1) constraint t2_pk primary key ,y date);
Table created.
zx@ORCL>insert into t2 values('5',sysdate);
1 row created.
zx@ORCL>commit;
Commit complete.
zx@ORCL>exec dbms_stats.gather_table_stats(USER,'T2',cascade=>true);
PL/SQL procedure successfully completed.
zx@ORCL>explain plan for select * from t2 where x=5;
Explained.
zx@ORCL>select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1513984157
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T2 | 1 | 12 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER("X")=5)
Note
-----
- dynamic sampling used for this statement (level=2)