其实object_id和data_object_id同样是表示数据库对象的一个唯一标志，但是object_id表示的是逻辑id，data_object_id表示的是物理id。如果一些object没有物理属性的话那它就不存在data_object_id，例如procedure,function,package,data type,db link,mv定义,view定义,临时表,分区表定义等等这些object都是没有对应着某个segment，因此它们的data_object_id都为空。

当表刚创建的时候它的object_id和data_object_id都是相等的，但是如果表经过move或truncate后那么data_object_id将会有变化。

SQL 10G>select object_id,data_object_id from user_objects where object_name='T';

OBJECT_ID DATA_OBJECT_ID
---------- --------------
63053 63053

刚开始创建表时object_id=data_object_id。

SQL 10G>SELECT HEADER_FILE,HEADER_BLOCK,BLOCKS FROM DBA_SEGMENTS WHERE SEGMENT_NAME='T' AND OWNER='TEST';

HEADER_FILE HEADER_BLOCK BLOCKS
----------- ------------ ----------
4 2019 8

SQL 10G>alter table t move;

Table altered.

SQL 10G>select object_id,data_object_id from user_objects where object_name='T';

OBJECT_ID DATA_OBJECT_ID
---------- --------------
63053 63463

在move以后可以看到data_object_id发生变化了。

SQL 10G>SELECT HEADER_FILE,HEADER_BLOCK,BLOCKS FROM DBA_SEGMENTS WHERE SEGMENT_NAME='T' AND OWNER='TEST';

HEADER_FILE HEADER_BLOCK BLOCKS
----------- ------------ ----------
4 467 8

SQL 10G>truncate table t;

Table truncated.

SQL 10G>select object_id,data_object_id from user_objects where object_name='T';

OBJECT_ID DATA_OBJECT_ID
---------- --------------
63053 63464

SQL 10G>SELECT HEADER_FILE,HEADER_BLOCK,BLOCKS FROM DBA_SEGMENTS WHERE SEGMENT_NAME='T' AND OWNER='TEST';

HEADER_FILE HEADER_BLOCK BLOCKS
----------- ------------ ----------
4 467 8

truncate之后虽然segment的位置没有移动，但是data_object_id还是发生变化了。

SQL 10G>alter table t add(b number);

Table altered.

SQL 10G>select object_id,data_object_id from user_objects where object_name='T';

OBJECT_ID DATA_OBJECT_ID
---------- --------------
63053 63464

我们对t表加一个字段，发现data_object_id没有发生变化，我们可以认为只有当segment发生变化时data_object_id才会随之变化。

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object_id和data_object_id

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