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)

可以看到，它会全面扫描表；另外即使我们对查询给出了以下提示：
zx@ORCL>explain plan for select /*+ index(t2 t2_pk) */ * from t2 where x=5;
?
Explained.
?
zx@ORCL>select * from table(dbms_xplan.display);
?
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3365102699
?
-------------------------------------------------------------------------------------
| Id? | Operation? ? ? ? ? ? ? ? ? | Name? | Rows? | Bytes | Cost (%CPU)| Time? ? |
-------------------------------------------------------------------------------------
|? 0 | SELECT STATEMENT? ? ? ? ? ? |? ? ? |? ? 1 |? ? 10 |? ? 2? (0)| 00:00:01 |
|? 1 |? TABLE ACCESS BY INDEX ROWID| T2? ? |? ? 1 |? ? 10 |? ? 2? (0)| 00:00:01 |
|*? 2 |? INDEX FULL SCAN? ? ? ? ? | T2_PK |? ? 1 |? ? ? |? ? 1? (0)| 00:00:01 |
-------------------------------------------------------------------------------------
?
Predicate Information (identified by operation id):
---------------------------------------------------
?
? 2 - filter(TO_NUMBER("X")=5)