已建索引执行计划为何不走索引？

DROP INDEX scott.IDX_UNI_EMP;

CREATE INDEX scott.IDX_EMP_1 ON scott.EMPLOYEE (EMPLOYEE_ID);

--

SELECT

*

FROM

scott.EMPLOYEE

WHERE

EMPLOYEE_ID = 100;

SELECT

*

FROM

TABLE (

DBMS_XPLAN.DISPLAY_CURSOR (NULL, NULL, 'ADVANCED')

);

--

TRUNCATE TABLE scott.EMPLOYEE;

BEGIN

FOR I IN 1 .. 5000 LOOP

INSERT INTO scott.EMPLOYEE

VALUES

('F', I) ;

END LOOP ;

COMMIT ;

END ;

/

BEGIN

FOR I IN 5001 .. 10000 LOOP

INSERT INTO scott.EMPLOYEE

VALUES

('M', I) ;

END LOOP ;

COMMIT ;

END ;

/

--

SELECT

GENDER,

COUNT (*)

FROM

scott.EMPLOYEE

GROUP BY

GENDER;

EXEC DBMS_STATS .GATHER_TABLE_STATS(ownname=>'scott',tabname=>'EMPLOYEE',estimate_percent=>100,CASCADE=>TRUE,method_opt=>'for all columns size 1',no_invalidate=>FALSE);

SET AUTOT TRACE

--

SELECT

EMPLOYEE_ID

FROM

scott.EMPLOYEE;

--强制也不走

SELECT

/*+ INDEX(EMPLOYEE IDX_EMP_1) */

EMPLOYEE_ID

FROM

scott.EMPLOYEE;

为什么？

索引IDX_EMP_1是个单键值B树索引，因此null值不会存储其中，但是一旦EMPLOYEE_ID有了null（尽管此实验没有null）,此索引会跳过null,导致执行结果不对，执行计划就有可能不会走,因此选择走全表扫描

处理办法：ALTER TABLE scott.EMPLOYEE MODIFY (EMPLOYEE_ID NOT NULL);