Oracle的数据字典表dba_objects包含了两个字段，object_id, data_object_id，官方文档上的解释是：
object_id: Dictionary object number of the object. Data_object_id: Dictionary object number of the segment that contains the object.
直译一下是：
object_id：对象的数据字典标示。 Data_object_id：包含对象的段的数据字典标示。
直译下来还是很难理解，不过下面就详细讲解他们的区别:
首先，区别一下段（segment）和数据字典对象（dictionary object）的概念，段（segment）是指实实在在的分配了一个或者多个区（extents）来存储数据。而数据字典对象（dictionary object）有可能有存储区域，也有可能没有。比如sequence，package，type这些对象并没有存储空间，所以并不存在段与之相关联。所以这些对象的data_object_id都是空值。
XPCHILD/XPCHILD@ORCL>select object_id, data_object_id from dba_objects where object_name=‘SEQ_TEST‘; OBJECT_ID DATA_OBJECT_ID ---------- -------------- 36385
下面看一个实际的例子，就能够很好的理解这两个概念了:
1. move 操作：
XPCHILD/XPCHILD@ORCL>select object_id, data_object_id from dba_objects where object_name=‘TEST1‘; OBJECT_ID DATA_OBJECT_ID ---------- -------------- 36386 36386 1 row selected. XPCHILD/XPCHILD@ORCL>alter table test1 move; Table altered. XPCHILD/XPCHILD@ORCL>select object_id, data_object_id from dba_objects where object_name=‘TEST1‘; OBJECT_ID DATA_OBJECT_ID ---------- -------------- 36386 36387
可以看到，test1在创建的时候，object_id, data_object_id都是36386.这是因为在创建的时候数据字典分配的机制是相同的。使用move操作，而本身move操作只是重新分配了空间来重组原来的数据，所以对象本身没有发生变化，而是重新分配了段来存储数据。
2. partition分区表
XPCHILD/XPCHILD@ORCL>CREATE TABLE test2 2 (id number, status char(1)) PARTITION BY list(status) (PARTITION p_t VALUES (‘t‘), PARTITION p_f VALUES (‘f‘)); Table created. XPCHILD/XPCHILD@ORCL>select object_name, SUBOBJECT_NAME, OBJECT_ID, DATA_OBJECT_ID from dba_objects where object_name=‘TEST2‘; OBJECT_NAM SUBOBJECT_NAME OBJECT_ID DATA_OBJECT_ID ---------- ------------------------------ ---------- -------------- TEST2 P_F 36390 36390 TEST2 P_T 36389 36389 TEST2 36388 3 rows selected
这个地方可以看出，test2对象只有object_id，而真正只有分区才会有data_object_id, 因为每一个分区分配了一个段。
接着再创建一个普通表：
XPCHILD/XPCHILD@ORCL>CREATE TABLE test3 ( id number, status char(1) ); XPCHILD/XPCHILD@ORCL>select object_name, object_id, data_object_id from dba_objects where object_name=‘TEST3‘; OBJECT_NAM OBJECT_ID DATA_OBJECT_ID ---------- ---------- -------------- TEST3 36391 36391
然后进行分区交换：
XPCHILD/XPCHILD@ORCL>alter table test2 exchange partition p_t with table test3 including indexes; Table altered.
最后再看一下相关的object_id和data_object_id：
XPCHILD/XPCHILD@ORCL>select object_name, SUBOBJECT_NAME, object_id, data_object_id from dba_objects where object_name in (‘TEST2‘,‘TEST3‘); OBJECT_NAM SUBOBJECT_NAME OBJECT_ID DATA_OBJECT_ID ---------- ------------------------------ ---------- -------------- TEST2 P_F 36390 36390 TEST2 P_T 36389 36391 TEST2 36388 TEST3 36391 36389
所以这个地方显而易见，在交换过后，对象的object_id都不会发生变变化，因为exchange操作并不会迁移数据，而仅仅是更改了对象的数据存储即段得指向，也就是更改了对象的指针，这个指针指向的是数据存储区域，即所谓的段。
3. 最后再看一下rowid
rowid是指数据块中行的物理地址。看一下rowid的组成： XPCHILD/XPCHILD@ORCL>select rowid , 2 substr(rowid,1,6) "OBJECT", 3 substr(rowid,7,3) "FILE", 4 substr(rowid,10,6) "BLOCK", 5 substr(rowid,16,3) "ROW", 6 dbms_rowid.rowid_object(rowid) object_id, 7 id 8 from test3 t; ROWID OBJECT FILE BLOCK ROW OBJECT_ID ID ------------------ ------------ ------ ------------ ------ ---------- ---------- AAAI4lAAMAAAhssAAA AAAI4l AAM AAAhss AAA 36389 1 1 row selected. XPCHILD/XPCHILD@ORCL>alter table test3 move; Table altered. XPCHILD/XPCHILD@ORCL>select rowid , 2 substr(rowid,1,6) "OBJECT", 3 substr(rowid,7,3) "FILE", 4 substr(rowid,10,6) "BLOCK", 5 substr(rowid,16,3) "ROW", 6 dbms_rowid.rowid_object(rowid) object_id, 7 id 8 from test3 t; ROWID OBJECT FILE BLOCK ROW OBJECT_ID ID ------------------ ------------ ------ ------------ ------ ---------- ---------- AAAI4oAAMAAAhtMAAA AAAI4o AAM AAAhtM AAA 36392 1 1 row selected.
可以看到，rowid表示的是数据的在块中的地址，所以在rowid的组成中，object_id是所谓的段的数据字典id，即data_object_id，如上所示，对表进行move操作，object_id发生了变化，即验证了这个猜想。