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。