Oracle隐式数据类型转换对索引的影响
对于oracle数据库来说，在条件允许的情况下，oracle可以帮我们自动做数据类型的转换，但是oracle还是建议我们最好使用转换函数（to_char()、to_date()、to_number()等）做显示的转换
下面这个例子简单的测试了，由于oracle自动执行隐式转换 ，导致索引无法正确使用的例子
建表T，其中ID列是字符类型
scott@DB01> create table t(id varchar2(10),name varchar2(10),sal number);
Table created.
描述表EMP，我们注意到empno是数字类型
scott@DB01> desc emp
Name Null? Type
----------------------------------------------------- -------- ------------------------------------
EMPNO NOT NULL NUMBER(4)
ENAME VARCHAR2(10)
JOB VARCHAR2(9)
MGR NUMBER(4)
HIREDATE DATE
SAL NUMBER(7,2)
COMM NUMBER(7,2)
DEPTNO NUMBER(2)
使用子查询插入插入数据到T表，我们其实应该意识到这里就涉及到数据类型的转换，T表的ID列是字符类型，虽然查询的结果看着是数字
scott@DB01> insert into t select empno,ename,sal from emp;
14 rows created.
scott@DB01> commit;
Commit complete.
scott@DB01> select * from t;
ID NAME SAL
---------- ---------- ----------
7369 SMITH 1000
7499 ALLEN 1600
7521 WARD 1250
7566 JONES 2975
7654 MARTIN 1250
7698 BLAKE 2850
7782 CLARK 2450
7788 SCOTT 3000
7839 KING 5000
7844 TURNER 1500
7876 ADAMS 1100
7900 JAMES 950
7902 FORD 3000
7934 MILLER 1300
14 rows selected.
在表T上创建索引T_IDX
scott@DB01> create index t_idx on t(id);
Index created.
scott@DB01> desc t
Name Null? Type
----------------------------------------------------- -------- ------------------------------------
ID VARCHAR2(10)
NAME VARCHAR2(10)
SAL NUMBER
很多人会想当然的认为，oracle既然可以帮我们做隐式数据类型的转换，那么下面两个语句的效果应该是一样的，如果光从结果来看确实如此。但实际上性能是有非常大的差异的
scott@DB01> select * from t where id=7369;
ID NAME SAL
---------- ---------- ----------
7369 SMITH 1000
scott@DB01> select * from t where id='7369';
ID NAME SAL
---------- ---------- ----------
7369 SMITH 1000
使用autotrace工具查看语句的执行计划，我们会发现第一条语句发生了数据类型的隐式转换，导致的结果是语句没有使用到索引T_IDX
scott@DB01> set autotrace trace exp
scott@DB01> select * from t where id=7369;
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 27 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 1 | 27 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER("ID")=7369)
Note
-----
- dynamic sampling used for this statement
scott@DB01>
scott@DB01> select * from t where id='7369';
Execution Plan
----------------------------------------------------------
Plan hash value: 470836197
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 27 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 27 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | T_IDX | 1 | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"='7369')
Note
-----
- dynamic sampling used for this statement