请记住：Oracle一般都会根据统计信息生成相对高效的执行计划，如果没有，那肯定是有理由的。不要慌乱，需要仔细分析原因。
测试数据：
SQL> select count(*) from t2;
COUNT(*)
----------
106688
SQL> create index i_t2_objectid on t2(object_id);
索引已创建。
SQL> analyze table t2 compute statistics for table for all indexes for all indexed columns;
表已分析。
SQL> desc t2;
名称 是否为空? 类型
----------------------------------------- -------- ----------------------------
OWNER VARCHAR2(30)
OBJECT_NAME VARCHAR2(128)
SUBOBJECT_NAME VARCHAR2(30)
OBJECT_ID VARCHAR2(100)
DATA_OBJECT_ID NUMBER
OBJECT_TYPE VARCHAR2(19)
CREATED DATE
LAST_DDL_TIME DATE
TIMESTAMP VARCHAR2(19)
STATUS VARCHAR2(7)
TEMPORARY VARCHAR2(1)
GENERATED VARCHAR2(1)
SECONDARY VARCHAR2(1)
NAMESPACE NUMBER
EDITION_NAME VARCHAR2(30)
SQL> set autot trace
varchar没有number的优先级高，需要先将varchar隐式类型转换成number，再和20进行比较。
隐式类型转换导致object_id字段不走索引。
SQL> select * from t2 where object_id=20;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 1513984157
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 744 | 411 (1)| 00:00:05 |
|* 1 | TABLE ACCESS FULL| T2 | 8 | 744 | 411 (1)| 00:00:05 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER("OBJECT_ID")=20)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
1465 consistent gets
0 physical reads
0 redo size
1536 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
SQL> /
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 1513984157
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 744 | 411 (1)| 00:00:05 |
|* 1 | TABLE ACCESS FULL| T2 | 8 | 744 | 411 (1)| 00:00:05 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER("OBJECT_ID")=20)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1465 consistent gets
0 physical reads
0 redo size
1536 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
将20显示转换成字符类型，object_id字段无需类型转换，用到索引。逻辑读为11，比全表扫描时1465下降的比例很大。
SQL> select * from t2 where object_id='20';
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 1119120461
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 744 | 9 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T2 | 8 | 744 | 9 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | I_T2_OBJECTID | 8 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"='20')
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
11 consistent gets
0 physical reads
0 redo size
1980 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
SQL> /
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 1119120461
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 744 | 9 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T2 | 8 | 744 | 9 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | I_T2_OBJECTID | 8 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"='20')
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
11 consistent gets
0 physical reads
0 redo size
1980 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
SQL>
另外，索引列使用函数（除非建了索引函数）、表达式、绑定变量窥测（由于执行计划共享会使用第一个传入值生成的执行计划，如果第一个生成的执行计划不使用索引，则之后就算传入值使用索引会更高效，也不会使用索引）、统计信息老旧等原因也会导致索引列不会用到索引，也需要注意可能根据索引列查询出的数据量较大，Oracle判断不走索引可能会高效也会忽略索引。在优化过程中，如果碰到索引列没有走索引，不要慌乱，Oracle对索引列不走索引都是有依据的，需要按实际情况进行分析。