Index是不存储NULL值的，所以如果你有NULL的条件在SQL中，那么相关的索引是用不到的。

CREATE TABLE PTIAN1 AS (SELECT * FROM DBA_OBJECTS);

CREATE INDEX PTIAN_INDEX ON PTIAN1(DATA_OBJECT_ID);

EXEC DBMS_STATS.GATHER_TABLE_STATS('APPS','PTIAN1', CASCADE=>TRUE );

SELECT * FROM PTIAN1 WHERE DATA_OBJECT_ID = 11;

SELECT * FROM PTIAN1 WHERE DATA_OBJECT_ID IS NULL;

解决方法

创建Function-Based Index 

CREATE INDEX PTIAN_FUNC_INDEX ON PTIAN1 (NVL(DATA_OBJECT_ID,-1));

EXEC DBMS_STATS.GATHER_TABLE_STATS('APPS','PTIAN1', CASCADE=>TRUE );

SELECT OBJECT_NAME FROM PTIAN1 WHERE  NVL(DATA_OBJECT_ID,-1) = -1;