为什么会产生这种问题：在执行含有绑定变量的查询语句时，完成解析和最优化操作之后对绑定变量进行绑定，这以为着在实现最优化操作时无法使用绑定变量列的统计信息。为了解决这个问题，数据库使用了窥探技术，在第一次解析SQL时，按照窥探变量的值生成执行计划，以后这样的SQL都按照这个执行。隐藏参数_optim_peek_user_binds=true则启用绑定变量窥探，否则CBO认为统计列是均匀的。下面来做个实验，注意的是 set autotrace这个玩意产生的执行计划可能不准：
SQL> create table test as select * from all_objects;
SQL> create index ind_object_name on TEST (object_name);
SQL> exec dbms_stats.gather_table_stats(user,'TEST',cascade=>true);
SQL> update test set object_name='aaa';
SQL> commit;
SQL> update test set object_name='bbb' where object_id=20;
SQL> commit;
SQL> select count(1) from test where object_name='aaa';
COUNT(1)
----------
50653
SQL> select count(1) from test where object_name='bbb';
COUNT(1)
----------
1
SQL> var ccc varchar2(10);
SQL> exec :ccc:='aaa';--按道理说是全表扫描，结果也是全表扫描
SQL> set autotrace exp
SQL> select * from test where object_name=:ccc;
SQL> set autotrace off;
SQL> select hash_value, child_number
2 from v$sql
3 where sql_text = 'select * from test where object_name=:ccc';
HASH_VALUE CHILD_NUMBER
---------- ------------
2373399249 0
SQL> select * from table(dbms_xplan.display_cursor(2373399249,0));
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 163 (100)| |
|* 1 | TABLE ACCESS FULL| TEST | 50654 | 3611K| 163 (4)| 00:00:02 |
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_NAME"=:CCC)
---------------------------------------------------------------------------------------------------------------------
SQL> exec :ccc:='bbb';--按道理说是索引扫描，但是结果却是全表扫描
SQL> set autotrace exp
SQL> select * from test where object_name=:ccc;
SQL> set autotrace off;
SQL> select hash_value, child_number
2 from v$sql
3 where sql_text = 'select * from test where object_name=:ccc';
HASH_VALUE CHILD_NUMBER
---------- ------------
2373399249 0
SQL> select * from table(dbms_xplan.display_cursor(2373399249,0));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 163 (100)| |
|* 1 | TABLE ACCESS FULL| TEST | 50654 | 3611K| 163 (4)| 00:00:02 |
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_NAME"=:CCC)
SQL> alter system flush shared_pool;--清理share_pool
SQL> exec :ccc:='bbb';--按道理说是索引扫描，结果也是索引扫描
SQL> set autotrace exp
SQL> select * from test where object_name=:ccc;
SQL> select hash_value, child_number
2 from v$sql
3 where sql_text = 'select * from test where object_name=:ccc';
HASH_VALUE CHILD_NUMBER
---------- ------------
2373399249 0
SQL> select * from table(dbms_xplan.display_cursor(2373399249,0));
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 73 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_OBJECT_NAME | 1 | |1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_NAME"=:CCC)
SQL> exec :ccc:='aaa';--按道理说是全表扫描，但结果却是索引扫描
SQL> select * from test where object_name=:ccc;
SQL> select hash_value, child_number
2 from v$sql
3 where sql_text = 'select * from test where object_name=:ccc';
HASH_VALUE CHILD_NUMBER
---------- ------------
2373399249 0
SQL> select * from table(dbms_xplan.display_cursor(2373399249,0));
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 73 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_OBJECT_NAME | 1 | |1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_NAME"=:CCC)
http://blog.csdn.net/stevendbaguo/article/details/8175633