oracle object_id和data_object_id的区别

时间:2014-04-28 04:23:13   收藏:0   阅读:725

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:

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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
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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

  

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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
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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:

  

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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
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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:

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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
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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

  

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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.
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  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。

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