what is data_object_id?

前些天有人问data_object_id究竟是个什么东东，既然有了object_id那还要data_object_id干吗？

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
―――- ――――�C
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
―――�C ―――― ―――-
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
―――- ――――�C
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
―――�C ―――― ―――-
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
―――- ――――�C
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
―――�C ―――― ―――-
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
―――- ――――�C
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。我们来看看data_object_id从哪里来的

SQL 10G> select text from dba_views where view_name=’DBA_OBJECTS’；

select u.name, o.name, o.subname, o.obj#, o.dataobj#,
decode(o.type#, 0, ‘NEXT OBJECT’, 1, ‘INDEX’, 2, ‘TABLE’, 3, ‘CLUSTER’,
4, ‘VIEW’, 5, ‘SYNONYM’, 6, ‘SEQUENCE’,
7, ‘PROCEDURE’, 8, ‘FUNCTION’, 9, ‘PACKAGE’,
11, ‘PACKAGE BODY’, 12, ‘TRIGGER’,
13, ‘TYPE’, 14, ‘TYPE BODY’,
19, ‘TABLE PARTITION’, 20, ‘INDEX PARTITION’, 21, ‘LOB’,
22, ‘LIBRARY’, 23, ‘DIRECTORY’, 24, ‘QUEUE’,
28, ‘JAVA SOURCE’, 29, ‘JAVA CLASS’, 30, ‘JAVA RESOURCE’,
32, ‘INDEXTYPE’, 33, ‘OPERATOR’,
34, ‘TABLE SUBPARTITION’, 35, ‘INDEX SUBPARTITION’,
40, ‘LOB PARTITION’, 41, ‘LOB SUBPARTITION’,
42, NVL((SELECT distinct ‘REWRITE EQUIVALENCE’
FROM sum$ s
WHERE s.obj#=o.obj#
and bitand(s.xpflags, 8388608) = 8388608),
‘MATERIALIZED VIEW’),
43, ‘DIMENSION’,
44, ‘CONTEXT’, 46, ‘RULE SET’, 47, ‘RESOURCE PLAN’,
48, ‘CONSUMER GROUP’,
51, ‘SUBSCRIPTION’, 52, ‘LOCATION’,
55, ‘XML SCHEMA’, 56, ‘JAVA DATA’,
57, ‘SECURITY PROFILE’, 59, ‘RULE’,
60, ‘CAPTURE’, 61, ‘APPLY’,
62, ‘EVALUATION CONTEXT’,
66, ‘JOB’, 67, ‘PROGRAM’, 68, ‘JOB CLASS’, 69, ‘WINDOW’,
72, ‘WINDOW GROUP’, 74, ‘SCHEDULE’, 79, ‘CHAIN’,
81, ‘FILE GROUP’,
‘UNDEFINED’),
o.ctime, o.mtime,
to_char(o.stime, ‘YYYY-MM-DD:HH24:MI:SS’),
decode(o.status, 0, ‘N/A’, 1, ‘VALID’, ‘INVALID’),
decode(bitand(o.flags, 2), 0, ‘N’, 2, ‘Y’, ‘N’),
decode(bitand(o.flags, 4), 0, ‘N’, 4, ‘Y’, ‘N’),
decode(bitand(o.flags, 16), 0, ‘N’, 16, ‘Y’, ‘N’)
from sys.obj$ o, sys.user$ u
where o.owner# = u.user#
and o.linkname is null
and (o.type# not in (1 /* INDEX - handled below */,
10 /* NON-EXISTENT */)
or
(o.type# = 1 and 1 = (select 1
from sys.ind$ i
where i.obj# = o.obj#
and i.type# in (1, 2, 3, 4, 6, 7, 9))))
and o.name != ‘_NEXT_OBJECT’
and o.name != ‘_default_auditing_options_’
union all
select u.name, l.name, NULL, to_number(null), to_number(null),
‘DATABASE LINK’,
l.ctime, to_date(null), NULL, ‘VALID’,'N’,'N’, ‘N’
from sys.link$ l, sys.user$ u
where l.owner# = u.user#

可以看到data_object_id是从obj$.dataobj#来的，而obj$.dataobj#又对应着seg$.HWMINCR

SQL 10G>select max(HWMINCR) from sys.seg$;

MAX(HWMINCR)
――――
63464

这里的HWMINCR就是data_object_id的来源，每次seg$里生成新的一条记录都会增加HWMINCR这个值，同时obj$.dataobj#也会跟着变化。