How to Find Free Space in Oracle table - database

Question

In the below script i am checking regularly the table size in my oracle database but i would like to be able to check free space on the database too.
Is there any way to add how much the free space is ? please
select user_segments.SEGMENT_NAME AS Table_Name,
user_segments.BYTES/1024/1024 AS Table_Size_MB,
my_indexes.Indexes_Size_MB AS Indexes_Size_MB,
((user_segments.BYTES/1024/1024) + my_indexes.Indexes_Size_MB) AS Tot_Size_MB,
u_tables.Num_Rows AS NUM_ROWS
from USER_SEGMENTS
inner join (
select
TABLE_NAME AS INDX_TABLE_NAME,
SUM(BYTES)/1024/1024 AS Indexes_Size_MB
from (
select
user_indexes.TABLE_NAME,
user_segments.SEGMENT_NAME,
user_segments.BYTES
from user_segments
inner join user_indexes ON user_segments.SEGMENT_NAME = user_indexes.INDEX_NAME
) group by TABLE_NAME
) my_indexes on my_indexes.INDX_TABLE_NAME = user_segments.SEGMENT_NAME
inner join (
select
TABLE_NAME AS USR_TABLE_NAME,
Num_Rows
from user_tables
) u_tables on u_tables.USR_TABLE_NAME = my_indexes.INDX_TABLE_NAME
order by TOT_SIZE_MB desc;

Answer 1

Here's an example of what you could do
Here's my table starting "clean"
SQL> create table t as
2 select d.* from dba_objects d,
3 ( select 1 from dual connect by level <= 20 );
Table created.
SQL>
SQL>
SQL> select num_rows, avg_row_len, blocks, empty_blocks
2 from user_tables
3 where table_name = 'T';
NUM_ROWS AVG_ROW_LEN BLOCKS EMPTY_BLOCKS
---------- ----------- ---------- ------------
1745660 131 33746 0
1 row selected.
Now I'll try see if I can get to the number using an estimate based on the stats I have
SQL> select num_rows*avg_row_len/8192*100/(100-pct_free) est_blocks
2 from user_tables
3 where table_name = 'T';
EST_BLOCKS
----------
31016.9081
1 row selected.
I'm close but a little bit off which is to be expected, because blocks have some overhead etc. But I can find out what the overhead is
SQL> select round(32300/29800,2) est_overhead from dual;
EST_OVERHEAD
------------
1.08
1 row selected.
So if I factor in that 8% into my calcs (for a clean table), I can now use the dictionary stats to get a good estimate of the expected blocks required for this table given the nunber of rows and their size is.
SQL> select num_rows*avg_row_len/8192*100/(100-pct_free)*1.08 est_blocks
2 from user_tables
3 where table_name = 'T';
EST_BLOCKS
----------
33498.2607
1 row selected.
Armed with this information, you can now do easy comparisons between what the size of a table is, versus what you would expect it to be based on the rows it contains
SQL> delete from t
2 where mod(object_id,3) = 0;
582000 rows deleted.
SQL>
SQL> exec dbms_stats.gather_table_stats('','T')
PL/SQL procedure successfully completed.
My calc suggests the table should be 22329 blocks but its actually 33746.
SQL> select blocks, num_rows*avg_row_len/8192*100/(100-pct_free)*1.08 est_blocks
2 from user_tables
3 where table_name = 'T';
BLOCKS EST_BLOCKS
---------- ----------
33746 22329.999
1 row selected.
Lets see how good the estimate was. I'll reorg the table to reclaim that space
SQL> alter table t move;
Table altered.
SQL>
SQL>
SQL> exec dbms_stats.gather_table_stats('','T')
PL/SQL procedure successfully completed.
SQL>
SQL>
SQL> select num_rows, avg_row_len, blocks, empty_blocks
2 from user_tables
3 where table_name = 'T';
NUM_ROWS AVG_ROW_LEN BLOCKS EMPTY_BLOCKS
---------- ----------- ---------- ------------
1163660 131 22536 0
1 row selected.
SQL>
So you can use a similar approach (and 8% is probably a good enough fudge factor)