[20190227]简单探究tab$的bojb#字段.txt

[20190227]简单探究tab$的bojb#字段.txt

--//上午做了删除tab$表,其对应索引i_tab1的恢复,我一直以为这个索引会很大,没有想到在我的测试环境仅仅139个键值.
--//查看/u01/app/oracle/product/11.2.0.4/dbhome_1/rdbms/admin/dcore.sql的内容tab$的定义如下.(我使用版本11.2.0.4)

1.环境:
SYS@book> @ ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

create table tab$                                             /* table table */
( obj#          number not null,                            /* object number */
  /* DO NOT CREATE INDEX ON DATAOBJ#  AS IT WILL BE UPDATED IN A SPACE
   * TRANSACTION DURING TRUNCATE */
  dataobj#      number,                          /* data layer object number */
  ts#           number not null,                        /* tablespace number */
  file#         number not null,               /* segment header file number */
  block#        number not null,              /* segment header block number */
  bobj#         number,                /* base object number (cluster / iot) */
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ...
  spare1        number,                       /* used to store hakan_kqldtvc */
  spare2        number,         /* committed partition # used by drop column */
  spare3        number,                           /* summary sequence number */
  spare4        varchar2(1000),         /* committed RID used by drop column */
  spare5        varchar2(1000),      /* summary related information on table */
  spare6        date                                  /* flashback timestamp */
)
cluster c_obj#(obj#)
/

--//可以从后面的注解发现base object number (cluster / iot).也就是IOT表以及cluster table的base object number.

2.对于cluster table:

select obj# from sys.tab$ where bobj#=2
minus
select object_id from dba_objects where data_object_id=2;

no rows selected

select object_id from dba_objects where data_object_id=2
minus
select obj# from sys.tab$ where bobj#=2

OBJECT_ID
----------
         2

SYS@book> select * from sys.tab$ where obj#=2  ;
no rows selected

SCOTT@book> select obj#,dataobj#,bobj#,tab# from sys.tab$ a where bobj#=2 order by obj#;
      OBJ#   DATAOBJ#      BOBJ#       TAB#
---------- ---------- ---------- ----------
         4          2          2          1
         5          2          2          2
        19          2          2          3
        20          2          2          4
        21          2          2          5
        80          2          2          6
        83          2          2          7
        86          2          2          8
        88          2          2          9
        92          2          2         10
        95          2          2         11
       114          2          2         12
       174          2          2         13
       252          2          2         14
       253          2          2         15
       512          2          2         16
       517          2          2         17
17 rows selected.

--//可以发现tab$表中没有obj#=2的记录.也就是对于cluster table,tab$的bobj#字段仅仅记录各个子表的情况.并且等于dataobj#.

3.IOT表的情况呢?
--//建立IOT看看:

SCOTT@book> create table t_iot ( a varchar2(10),b varchar2(10),vc varchar2(1200), constraint t_iot_pk PRIMARY KEY(a)) ORGANIZATION INDEX;
Table created.

SCOTT@book> select object_id,data_object_id,object_name from dba_objects where object_name in ('T_IOT_PK','T_IOT') and owner=user;
 OBJECT_ID DATA_OBJECT_ID OBJECT_NAME
---------- -------------- --------------------
     91110                T_IOT
     91111          91111 T_IOT_PK

--//对于IOT表的本质实际上是一个索引,仅仅索引段有空间分配(DATA_OBJECT_ID非空).

SCOTT@book> select obj#,dataobj#,bobj# from sys.tab$ a where obj# in (91110,91111);
      OBJ#   DATAOBJ#      BOBJ#
---------- ---------- ----------
     91110
--//表IOT没有任何段分配.奇怪没有obj#=91111的段.没有插入记录吗?

SCOTT@book> insert into t_iot values ('1','a','a');
1 row created.

SCOTT@book> commit ;
Commit complete.

SCOTT@book> select obj#,dataobj#,bobj# from sys.tab$ a where obj# in (91110,91111);
      OBJ#   DATAOBJ#      BOBJ#
---------- ---------- ----------
     91110

--//当然也很好理解OBJECT_ID=91111是索引段,不会出现在表tab$里面.如何理解注解base object number (cluster / iot)呢?
--//也就是索引组织表(IOT)什么时候会出现表段呢?难道是OVERFLOW段吗?建立包含overflow段的iot表看看:

SCOTT@book> create table z_iot ( a varchar2(10),b varchar2(10),vc varchar2(1200), constraint z_iot_pk PRIMARY KEY(a)) ORGANIZATION INDEX OVERFLOW  TABLESPACE users;
Table created.

SCOTT@book> select object_id,data_object_id,object_name from dba_objects where object_name in ('Z_IOT_PK','Z_IOT') and owner=user;
 OBJECT_ID DATA_OBJECT_ID OBJECT_NAME
---------- -------------- --------------------
     91112                Z_IOT
     91114          91114 Z_IOT_PK
--//注意1个细节,object_id出现跳号.

SCOTT@book> select * from dba_objects where  object_id between 91112 and 91114;
OWNER  OBJECT_NAME          SUBOBJECT_  OBJECT_ID DATA_OBJECT_ID OBJECT_TYPE CREATED             LAST_DDL_TIME       TIMESTAMP           STATUS  T G S  NAMESPACE EDITION_NAME
------ -------------------- ---------- ---------- -------------- ----------- ------------------- ------------------- ------------------- ------- - - - ---------- ------------
SCOTT  Z_IOT_PK                             91114          91114 INDEX       2019-02-28 15:51:44 2019-02-28 15:51:44 2019-02-28:15:51:44 VALID   N N N          4
SCOTT  SYS_IOT_OVER_91112                   91113          91113 TABLE       2019-02-28 15:51:44 2019-02-28 15:51:44 2019-02-28:15:51:44 VALID   N Y N          1
SCOTT  Z_IOT                                91112                TABLE       2019-02-28 15:51:44 2019-02-28 15:51:44 2019-02-28:15:51:44 VALID   N N N          1
--//可以看出多了一个对象SYS_IOT_OVER_91112就是OVERFLOW段(类型是table),也就是这个溢出段按照表的形式保持信息,对于这些东西
--//很少探究.正常的业务表很少使用IOT,至少国内的情况如此.

SCOTT@book> select obj#,dataobj#,bobj# from sys.tab$ a where obj# between 91112 and 91114;
      OBJ#   DATAOBJ#      BOBJ#
---------- ---------- ----------
     91112                 91113
     91113      91113      91112

--//注意看它们之间的关系.
--//OBJ#=91112,对应是表Z_IOT,没有数据段分配,DATAOBJ#等于null,BOBJ#=91113,对应是SYS_IOT_OVER_91112(后面的数字与Z_IOT的object_id一致)
--//OBJ#=91113,对应的是SYS_IOT_OVER_91112.DATAOBJ#=91113.BOBJ#=91112,对应的是Z_IOT,这也就是注解讲base object number(cluster / iot).
--//有点绕,大家慢慢理解吧...

--//这也就是sys.tab$表为什么bobj#非空的记录很少的原因,这样前面的修复索引成为可能,相对容易的缘故.