Oracle Segment不等于Extents？

Question

对于给定的表空间，为什么dba_extents中的字节总和不等于dba_segments中的字节总和？(示例脚本后的其他问题。)

SQL> with
  "SEG" as
    ( select 'segment_bytes' what
             , to_char(sum(bytes), '9,999,999,999,999') bytes
        from dba_segments
       where tablespace_name = 'MYDATA'
    )
  , "EXT" as
    ( select 'extent_bytes' what
             , to_char(sum(bytes), '9,999,999,999,999') bytes
        from dba_extents
       where tablespace_name = 'MYDATA'
    )
  , "FS" as
    ( select tablespace_name
             , sum(bytes) free_bytes
        from dba_free_space
       where tablespace_name = 'MYDATA'
       group by tablespace_name
    ),
  "DF" as
    ( select tablespace_name
             , sum(bytes) alloc_bytes
             , sum(user_bytes) user_bytes
        from dba_data_files
       where tablespace_name = 'MYDATA'
       group by tablespace_name
    )
  select what, bytes from SEG
  union all select 'datafile_bytes-freespace' what
                   , to_char(alloc_bytes - nvl(free_bytes, 0), '9,999,999,999,999') used_file_bytes
              from DF
                  left join FS
                         on DF.tablespace_name = FS.tablespace_name
  union all select 'datafile_userbytes-freespace' what
                   , to_char(user_bytes - nvl(free_bytes, 0), '9,999,999,999,999') used_user_bytes
             from DF
                  left join FS
                         on DF.tablespace_name = FS.tablespace_name
  union all select what, bytes from EXT
;

WHAT                         BYTES            
---------------------------- ------------------
segment_bytes                 2,150,514,819,072
datafile_bytes-freespace      2,150,528,540,672
datafile_userbytes-freespace  2,150,412,845,056
extent_bytes                  2,150,412,845,056

4 rows selected.

我原本期望segment_bytes等于extent_bytes或datafile_bytes-freespace，但它介于两者之间。

由于段“开销”(跟踪所有区段)，segment_bytes是否比extent_bytes更多？

如果是这样，那么这个段“开销”是否也是数据文件“开销”的一部分？

Oracle 19.1企业版。提前谢谢。

Answer 1

例如，dba_segments和dba_extents之间的区别可能在于来自recyclebin的对象:请查看我的测试数据库中的结果：

with 
 seg as (
    select segment_name,sum(bytes) b1 
    from dba_segments 
    group by segment_name
)
,ext as (
    select segment_name,sum(bytes) b2 
    from dba_extents 
    group by segment_name
)
select
   seg.segment_name seg1
  ,ext.segment_name seg2
  ,b1,b2
from seg full outer join ext on seg.segment_name=ext.segment_name
where lnnvl(b1=b2)
order by 1,2;

结果：

SEG1                           SEG2                                   B1         B2
------------------------------ ------------------------------ ---------- ----------
BIN$xi7yNJwFcIrgUwIAFaxDaA==$0                                     65536
BIN$xi7yNJwGcIrgUwIAFaxDaA==$0                                     65536
_SYSSMU10_2262159254$          _SYSSMU10_2262159254$                   0    4325376
_SYSSMU1_3588498444$           _SYSSMU1_3588498444$                    0    3276800
_SYSSMU2_2971032042$           _SYSSMU2_2971032042$                    0    2228224
_SYSSMU3_3657342154$           _SYSSMU3_3657342154$                    0    2228224
_SYSSMU4_811969446$            _SYSSMU4_811969446$                     0    2293760
_SYSSMU5_3018429039$           _SYSSMU5_3018429039$                    0    3276800
_SYSSMU6_442110264$            _SYSSMU6_442110264$                     0    2228224
_SYSSMU7_2728255665$           _SYSSMU7_2728255665$                    0    2097152
_SYSSMU8_801938064$            _SYSSMU8_801938064$                     0    2228224
_SYSSMU9_647420285$            _SYSSMU9_647420285$                     0    3276800

12 rows selected.

正如你所看到的，前两行是来自recyclebin的对象，所以你可以运行相同的查询，检查你的对象是否也在recyclebin中。它们在dba_extents中不可见，因为它们被segment_flag过滤掉了

select text_vc from dba_views where view_name='DBA_EXTENTS';

select ds.owner, ds.segment_name, ds.partition_name, ds.segment_type,
       ds.tablespace_name,
       e.ext#, f.file#, e.block#, e.length * ds.blocksize, e.length, e.file#
from sys.uet$ e, sys.sys_dba_segs ds, sys.file$ f
where e.segfile# = ds.relative_fno
  and e.segblock# = ds.header_block
  and e.ts# = ds.tablespace_id
  and e.ts# = f.ts#
  and e.file# = f.relfile#
  and bitand(NVL(ds.segment_flags,0), 1) = 0
  and bitand(NVL(ds.segment_flags,0), 65536) = 0
union all
select
       ds.owner, ds.segment_name, ds.partition_name, ds.segment_type,
       ds.tablespace_name,
       e.ktfbueextno, f.file#, e.ktfbuebno,
       e.ktfbueblks * ds.blocksize, e.ktfbueblks, e.ktfbuefno
from sys.sys_dba_segs ds, sys.x$ktfbue e, sys.file$ f
where e.ktfbuesegfno = ds.relative_fno
  and e.ktfbuesegbno = ds.header_block
  and e.ktfbuesegtsn = ds.tablespace_id
  and ds.tablespace_id = f.ts#
  and e.ktfbuefno = f.relfile#
  and bitand(NVL(ds.segment_flags, 0), 1) = 1
  and bitand(NVL(ds.segment_flags,0), 65536) = 0;

因此，如果我们注释掉这些谓词(bitand(NVL(segment_flags,0)....)并检查我们的差异(BIN$...和_SYSSMU...对象)，我们将发现哪些谓词过滤掉了它们：

with
 my_dba_extents(
   OWNER,SEGMENT_NAME,PARTITION_NAME
  ,SEGMENT_TYPE,TABLESPACE_NAME,EXTENT_ID,FILE_ID
  ,BLOCK_ID,BYTES,BLOCKS,RELATIVE_FNO
  ,segment_flags) 
as (
select ds.owner, ds.segment_name, ds.partition_name, ds.segment_type,
       ds.tablespace_name,
       e.ext#, f.file#, e.block#, e.length * ds.blocksize, e.length, e.file#
       ,segment_flags
from sys.uet$ e, sys.sys_dba_segs ds, sys.file$ f
where e.segfile# = ds.relative_fno
  and e.segblock# = ds.header_block
  and e.ts# = ds.tablespace_id
  and e.ts# = f.ts#
  and e.file# = f.relfile#
--  and bitand(NVL(ds.segment_flags,0), 1) = 0
--  and bitand(NVL(ds.segment_flags,0), 65536) = 0
union all
select
       ds.owner, ds.segment_name, ds.partition_name, ds.segment_type,
       ds.tablespace_name,
       e.ktfbueextno, f.file#, e.ktfbuebno,
       e.ktfbueblks * ds.blocksize, e.ktfbueblks, e.ktfbuefno
       ,segment_flags
from sys.sys_dba_segs ds, sys.x$ktfbue e, sys.file$ f
where e.ktfbuesegfno = ds.relative_fno
  and e.ktfbuesegbno = ds.header_block
  and e.ktfbuesegtsn = ds.tablespace_id
  and ds.tablespace_id = f.ts#
  and e.ktfbuefno = f.relfile#
--  and bitand(NVL(ds.segment_flags, 0), 1) = 1
--  and bitand(NVL(ds.segment_flags,0), 65536) = 0
)
select 
   segment_name
  ,bitand(NVL(segment_flags, 0), 1) as predicate_1
  ,bitand(NVL(segment_flags,0), 65536) as predicate_2
  ,case when bitand(NVL(segment_flags,0), 1) = 0     then 'y' else 'n' end pred_1_res
  ,case when bitand(NVL(segment_flags,0), 65536) = 0 then 'y' else 'n' end pred_2_res
from my_dba_extents e
where e.segment_name like 'BIN%'
   or e.segment_name like '_SYSSMU%';

SEGMENT_NAME                   PREDICATE_1 PREDICATE_2 PRED_1_RES     PRED_2_RES
------------------------------ ----------- ----------- -------------- --------------
_SYSSMU1_3588498444$                     1           0 n              y
_SYSSMU1_3588498444$                     1           0 n              y
_SYSSMU1_3588498444$                     1           0 n              y
_SYSSMU1_3588498444$                     1           0 n              y
_SYSSMU1_3588498444$                     1           0 n              y
_SYSSMU2_2971032042$                     1           0 n              y
_SYSSMU2_2971032042$                     1           0 n              y
...
_SYSSMU10_2262159254$                    1           0 n              y
_SYSSMU10_2262159254$                    1           0 n              y
_SYSSMU10_2262159254$                    1           0 n              y
BIN$xi7yNJwGcIrgUwIAFaxDaA==$0           1       65536 n              n
BIN$xi7yNJwFcIrgUwIAFaxDaA==$0           1       65536 n              n

Re " datafile _bytes-freespace"：别忘了每个数据文件都有自己的头，所以nor dba_segments，nor dba_extents都不应该计算它。

PS。其他10行是撤销段，但这不是您的情况，因为您的查询只检查MYDATA表空间，而不检查UNDO。