列定义允许null，可能导致Index Hint提升不走索引

遇到一个SB问题，加index hint提升，居然也不走索引，郁闷良久，结果发现该表的定义允许null，这对优化器有影响

SQL> desc emp
 名称                                                                                                      是否为空? 类型
 ----------------------------------------------------------------------------------------------------------------- -------- --------------

 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)
我在ename上建立了一个index

SQL> select table_name, index_name,column_name from user_ind_columns;

TABLE_NAME                     INDEX_NAME                     COLUMN_NAME
------------------------------ ------------------------------ -----------------
DEPT                           PK_DEPT                        DEPTNO
EMP                            PK_EMP                         EMPNO
EMP                            EMA_ENAME                      ENAME
SQL> select dbms_metadata.get_ddl('INDEX','EMA_ENAME','SCOTT') FROM DUAL;

DBMS_METADATA.GET_DDL('INDEX','EMA_ENAME','SCOTT')
--------------------------------------------------------------------------------

  CREATE INDEX "SCOTT"."EMA_ENAME" ON "SCOTT"."EMP" ("ENAME")
  PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
  TABLESPACE "USERS"

SQL> select /*+ index(emp.ename) */ ename from emp;

已选择14行。

执行计划
----------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |    84 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |    84 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------
可以看见即使提升了优化器走索引，结果优化器也不走

SQL> alter table emp modify ename not null;

表已更改。

SQL> select /*+ index(emp.ename) */ ename from emp ;

已选择14行。

执行计划
----------------------------------------------------------
Plan hash value: 1224545206

------------------------------------------------------------------------------
| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |           |    14 |    84 |     1   (0)| 00:00:01 |
|   1 |  INDEX FULL SCAN | EMA_ENAME |    14 |    84 |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------
现在将列定义为非null，用hint提升优化器就走了索引了。