Oracle的隐式转换

都说Oracle存在NUMBER和VARCHAR2类型的隐式转换，严格意义上需要避免，但为何需要避免，从下面的实验进行验证。

1. 创建测试表和索引
create table tn (id number, name varchar2(1));

create index idx_tn on tn (id);

create index idx_tn on tn (name);
分别对NUMBER类型的id字段，VARCHAR2类型的name字段创建索引。

2. 查看VARCHAR2->NUMBER的隐式转换

SQL> select * from tn where id = 1;

no rows selected

Execution Plan

----------------------------------------------------------

Plan hash value: 3532270966

---------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

---------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 13 | 1 (0)| 00:00:01 |

|* 1 | INDEX RANGE SCAN| IDX_TN | 1 | 13 | 1 (0)| 00:00:01 |

---------------------------------------------------------------------------

"where id = 1"用的是列索引范围扫描。

SQL> select * from tn where id = '123';

no rows selected

Execution Plan

----------------------------------------------------------

Plan hash value: 3532270966

---------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

---------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 13 | 1 (0)| 00:00:01 |

|* 1 | INDEX RANGE SCAN| IDX_TN | 1 | 13 | 1 (0)| 00:00:01 |

---------------------------------------------------------------------------

"where id = '123'"，Oracle会将字符类型的123转换为NUMBER类型进行比较，此处仍可使用索引范围扫描，说明VARCHAR2->NUMBER的隐式转换，未对索引产生影响。

3. 查看NUMBER->VARCHAR2的隐式转换

SQL> select * from tn where name = '123';

no rows selected

Execution Plan

----------------------------------------------------------

Plan hash value: 479240418

-------------------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

-------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 15 | 1 (0)| 00:00:01 |

| 1 | TABLE ACCESS BY INDEX ROWID| TN | 1 | 15 | 1 (0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | IDX_TN_NAME | 1 | | 1 (0)| 00:00:01 |

-------------------------------------------------------------------------------------------

"where name = '123'"使用的是索引范围扫描。

SQL> select * from tn where name = 123;

no rows selected

Execution Plan

----------------------------------------------------------

Plan hash value: 2655062619

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 15 | 2 (0)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| TN | 1 | 15 | 2 (0)| 00:00:01 |

--------------------------------------------------------------------------
"where name = 123"，Oracle会将数值类型的123转换为VARCHAR2字符类型，和name进行比较，此处用了全表扫描，说明name的列索引失效。

总结：
1. NAME和VARCHAR2之间可以进行隐式转换，其中VARCHAR2->NUMBER不会导致索引失效，NUMBER->VARCHAR2会让索引失效，因此这种隐式转换，是需要注意避免。
2. 之所以VARCHAR2->NUMBER不会让索引失效，我猜测是转换为where id = to_number('123')。NUMBER->VARCHAR2会让索引失效，我猜测是转换为where to_number(name) = 123。
3. 引申知识点，之所以上面id和name使用的是索引范围扫描，是因为建立的是非唯一B树索引，如果是unique索引，则会使用UNIQUE INDEX SCAN的扫描方式。

补充：
经lhrbest的指正，从谓词条件即可看出端倪。
​附：
​SQL> select * from tn where id = '123';
​Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("ID"=123)
​
​SQL> select * from tn where name = 123;
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(TO_NUMBER("NAME")=123)
​可以看出此处对NAME做了TO_NUMBER转换，导致索引失效。