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  • [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.

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  • 原文地址:https://www.cnblogs.com/lfree/p/10451183.html
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