Free space discrepancy

Question

Hi,
For a particular tablespace I am querying dba_segments view to calculate the utilized space size of that tablespace:
SQL> select tablespace_name,sum(bytes)/1024/1024/1024 gb
from dba_segments
where tablespace_name='TBS1'
group by tablespace_name;
The output is: *74.48*
To find out the total allocated size of this tablespace I have used this:
SQL>select tablespace_name, sum(bytes)/1024/1024/1024 gb
from dba_data_files
where tablespace_name='TBS1'
group by tablespace_name;
The output is: *77.43*
Then I queried dba_free_space to find out the free space of that tablespace using this:
SQL> select tablespace_name,sum(bytes)/1024/1024/1024 gb
from dba_free_space
where tablespace_name='TBS1'
group by tablespace_name;
The output is: *21.8*
Now here I am not able to understand that, why dba_free_space returned 21.8 GB as free space instead of 2.95 GB (calculated by subtracting the value returned by dba_segments from dba_data_files).
DB Version: 10.2.0.1, 64 bit
OS Platform: Linux
Thanks for your time.
Regards,
Joy

Answer 1

Hi,
Here i will give the approach how to troubleshoot this kind of issue.
find the dba_segments definination:
======================
SQL> set long 100000
SQL> select dbms_metadata.get_ddl('VIEW','DBA_SEGMENTS','SYS') FROM DUAL;
CREATE OR REPLACE FORCE VIEW "SYS"."DBA_SEGMENTS" ("OWNER", "SEGMENT_NAME", "P
ARTITION_NAME", "SEGMENT_TYPE", "TABLESPACE_NAME", "HEADER_FILE", "HEADER_BLOCK"
, "BYTES", "BLOCKS", "EXTENTS", "INITIAL_EXTENT", "NEXT_EXTENT", "MIN_EXTENTS",
"MAX_EXTENTS", "PCT_INCREASE", "FREELISTS", "FREELIST_GROUPS", "RELATIVE_FNO", "
BUFFER_POOL") AS
select owner, segment_name, partition_name, segment_type, tablespace_name,
header_file, header_block,
decode(bitand(segment_flags, 131072), 131072, blocks,
(decode(bitand(segment_flags,1),1,
dbms_space_admin.segment_number_blocks(tablespace_id, relative_fno,
header_block, segment_type_id, buffer_pool_id, segment_flags,
segment_objd, blocks), blocks)))*blocksize,
decode(bitand(segment_flags, 131072), 131072, blocks,
(decode(bitand(segment_flags,1),1,
dbms_space_admin.segment_number_blocks(tablespace_id, relative_fno,
header_block, segment_type_id, buffer_pool_id, segment_flags,
segment_objd, blocks), blocks))),
decode(bitand(segment_flags, 131072), 131072, extents,
(decode(bitand(segment_flags,1),1,
dbms_space_admin.segment_number_extents(tablespace_id, relative_fno,
header_block, segment_type_id, buffer_pool_id, segment_flags,
segment_objd, extents) , extents))),
initial_extent, next_extent, min_extents, max_extents, pct_increase,
freelists, freelist_groups, relative_fno,
decode(buffer_pool_id, 0, 'DEFAULT', 1, 'KEEP', 2, 'RECYCLE', NULL)
from sys_dba_segs
find the dba_free-space definination:
======================
SQL>
SQL> select dbms_metadata.get_ddl('VIEW','DBA_FREE_SPACE','SYS') FROM DUAL;
CREATE OR REPLACE FORCE VIEW "SYS"."DBA_FREE_SPACE" ("TABLESPACE_NAME", "FILE_
ID", "BLOCK_ID", "BYTES", "BLOCKS", "RELATIVE_FNO") AS
select ts.name, fi.file#, f.block#,
f.length * ts.blocksize, f.length, f.file#
from sys.ts$ ts, sys.fet$ f, sys.file$ fi
where ts.ts# = f.ts#
and f.ts# = fi.ts#
and f.file# = fi.relfile#
and ts.bitmapped = 0
union all
select /*+ ordered use_nl(f) use_nl(fi) */
ts.name, fi.file#, f.ktfbfebno,
f.ktfbfeblks * ts.blocksize, f.ktfbfeblks, f.ktfbfefno
from sys.ts$ ts, sys.x$ktfbfe f, sys.file$ fi
where ts.ts# = f.ktfbfetsn
and f.ktfbfetsn = fi.ts#
and f.ktfbfefno = fi.relfile#
and ts.bitmapped <> 0 and ts.online$ in (1,4) and ts.contents$ = 0
union all
select /*+ ordered use_nl(u) use_nl(fi) */
ts.name, fi.file#, u.ktfbuebno,
u.ktfbueblks * ts.blocksize, u.ktfbueblks, u.ktfbuefno
from sys.recyclebin$ rb, sys.ts$ ts, sys.x$ktfbue u, sys.file$ fi
where ts.ts# = rb.ts#
and rb.ts# = fi.ts#
and u.ktfbuefno = fi.relfile#
and u.ktfbuesegtsn = rb.ts#
and u.ktfbuesegfno = rb.file#
and u.ktfbuesegbno = rb.block#
and ts.bitmapped <> 0 and ts.online$ in (1,4) and ts.contents$ = 0
union all
select ts.name, fi.file#, u.block#,
u.length * ts.blocksize, u.length, u.file#
from sys.ts$ ts, sys.uet$ u, sys.file$ fi, sys.recyclebin$ rb
where ts.ts# = u.ts#
and u.ts# = fi.ts#
and u.segfile# = fi.relfile#
and u.ts# = rb.ts#
and u.segfile# = rb.file#
and u.segblock# = rb.block#
and ts.bitmapped = 0
DBA_FREE_SPACE is taking value from sys.recyclebin$ rb, sys.ts$ ts, sys.x$ktfbue u, sys.file$ fi
DBA_SGEMNETS taking value from sys_dba_segs
Solution:
=====
1) How the value will be populated to these view ?
When you collect the stats then the value will be populated to those table.
2) From where these view DBS_SEGMENTS derived ?
From base tables
So, Please collect the stats on fixed table then let me know the result
Kind Regards,
Rakesh jayappa