ORACLE左外部联接问题（带有空表？）

我想知道为什么这个查询：

select l.objectID LEFT_CODE, r.object_code RIGHT_CODE
from OGGETTO_ORGANIZZATIVO l left outer join
    ANAG_OGGETTO_ORGANIZZATIVO r on l.objectID = r.objectID
WHERE r.objectID IS NULL;

返回一些行，而另一个查询

select l.objectID LEFT_CODE
from OGGETTO_ORGANIZZATIVO l left outer join
    ANAG_OGGETTO_ORGANIZZATIVO r on l.objectID = r.objectID
WHERE r.objectID IS NULL;

不返回任何内容。我的问题是在右表中插入出现在左表中但不在右表中的所有行；目前右表为空。

看来，如果我没有在select子句中引用正确的字段，那么oracle将不会管理外部联接。

提前致谢。保罗

[UPDATE] 第一次查询

select l.codice_oggetto_SAP
from oggetto_organizzativo l left outer join
  ANAG_OGGETTO_ORGANIZZATIVO_SAP r
    ON l.codice_oggetto_sap = r.codice_oggetto_SAP
WHERE r.codice_oggetto_sap is null

执行计划

第二个查询

select l.codice_oggetto_SAP, r.codice_oggetto_SAP
from oggetto_organizzativo l left outer join
  ANAG_OGGETTO_ORGANIZZATIVO_SAP r
    ON l.codice_oggetto_sap = r.codice_oggetto_SAP
WHERE r.codice_oggetto_sap is null

执行计划

在(NULL IS NOT NULL)我看来，第一份执行计划声明似乎很好奇...

[第二次更新] @toddlermenot，

但是为什么这个工作呢？

CREATE TABLE LEFT_TBL
   (    "OBJECTID" VARCHAR2(20 BYTE)  ) ;

CREATE TABLE RIGHT_TBL
   (    "OBJECTID" VARCHAR2(20 BYTE)  ) ;

INSERT ALL
  INTO LEFT_TBL (OBJECTID) VALUES ('AAA')
  INTO LEFT_TBL (OBJECTID) VALUES ('BBB')
  INTO LEFT_TBL (OBJECTID) VALUES ('CCC')  
SELECT * from DUAL;

SELECT l.objectID
FROM LEFT_TBL l LEFT OUTER JOIN RIGHT_TBL r
  ON L.OBJECTID = r.OBJECTID
WHERE r.OBJECTID IS NULL;

这个执行计划

Plan hash value: 2059691840

--------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           |     3 |    72 |     5   (0)| 00:00:01 |
|*  1 |  HASH JOIN ANTI    |           |     3 |    72 |     5   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| LEFT_TBL  |     3 |    36 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| RIGHT_TBL |     1 |    12 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("L"."OBJECTID"="R"."OBJECTID")

Note
-----
   - dynamic sampling used for this statement (level=2)

可能是和索引问题？