类型转换导致执行计划不走索引测试案例

测试环境模拟：

SQL> drop table t_col_type purge;

create table t_col_type(id varchar2(20),col2 varchar2(20),col3 varchar2(20));

insert into t_col_type select rownum,'abc','efg' from dual connect by level<=10000;

commit;

create index idx_id on t_col_type(id);

set linesize 1000

set autotrace traceonlydrop table t_col_type purge

           *

ERROR at line 1:

ORA-00942: table or view does not exist

 

SQL> select * from t_col_type where id=6;

Execution Plan

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

Plan hash value: 3191204463

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

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

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

|   0 | SELECT STATEMENT              |                        |     1    |    36 |     8   (0)| 00:00:01 |

|*  1 |  TABLE ACCESS FULL            | T_COL_TYPE |     1    |    36 |     8   (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

   1 - filter(TO_NUMBER("ID")=6)

Note

-----

   - dynamic sampling used for this statement

Statistics

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

          5  recursive calls

          0  db block gets

         64  consistent gets

          0  physical reads

          0  redo size

        640  bytes sent via SQL*Net to client

        469  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

这里好像有点奇怪啊，明明建了index [create index idx_id on t_col_type(id);]但是为啥没有用到呢？

---查看表上列是否有索引

SQL> select index_name , table_name,column_name from all_ind_columns where table_name ='T_COL_TYPE';

INDEX_NAME

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

TABLE_NAME

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

COLUMN_NAME

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

IDX_ID

T_COL_TYPE

ID

----查看表结构

SQL> desc scott.T_COL_TYPE

 Name                                      Null?    Type

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

 ID                                                 VARCHAR2(20)----------注意这里的字符类型

 COL2                                               VARCHAR2(20)

 COL3                                               VARCHAR2(20)

再次关注下 执行计划中的谓语信息：

1 - filter(TO_NUMBER("ID")=6)  ----------这里发生了类型转换

所以在执行计划中就无法用已有的索引，那么如何才能让他正确走索引呢？

 

select * from t_col_type where id='6';------注意下这里的区别加了单引号，表明这是个字符，

Execution Plan

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

Plan hash value: 3998173245

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

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

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

|   0 |  SELECT STATEMENT                         |                       |     1 |    36 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID   | T_COL_TYPE |     1 |    36 |     2   (0)| 00:00:01 |

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

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

Predicate Information (identified by operation id):

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

   2 - access("ID"='6')

Note

-----

   - dynamic sampling used for this statement

Statistics

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

          9  recursive calls

          0  db block gets

         39  consistent gets

          1  physical reads

          0  redo size

        640  bytes sent via SQL*Net to client

        469  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed