Table free space

Question

Is there any way to find the amount of free space that will be created when we delete data from any table.
Suppose we delete 10,000 rows from a table, how much free space will be created ?

Answer 1

I wish to mention as below : (Oracle 10.2.0.1 on windows XP Prof Service Pack 2)
select ts.tablespace_name, to_char(sum(nvl(fs.bytes,0)), '999999999999999') as MB_FREE
  from dba_free_space fs, dba_tablespaces ts
  where fs.tablespace_name(+) = ts.tablespace_name
  group by ts.tablespace_name;
TABLESPACE_NAME                MB_FREE
USERS                                 280231936
Then I created one table in scott schema (which is having privilege on users tbs)
create table t(name varchar2(20),age number(3));
Now check space again :
SYS@orcl> /
TABLESPACE_NAME                MB_FREE
USERS                                 280166400
Means 280231936-280166400=65536 bytes consumed above create table command.  Why?
SCOTT@orcl> select bytes,blocks,extents from user_segments where segment_name='T';
              BYTES              BLOCKS             EXTENTS
           65536.00                8.00                1.00
But why it taken 65536 bytes?
Yes it will take 65536 bytes because my db_block_size=8192 bytes and by default it will create 1 extent of 8 blocks.  It has not occupied all 65536 bytes, but has been allocated for the table; means if insert 65536 bytes data there will be only 1 extent (if we don’t say alter table t allocate extent)  Now suppose if I said alter table t allocate extent; then :
SCOTT@orcl> select bytes,blocks,extents from user_segments where segment_name='T';
              BYTES              BLOCKS             EXTENTS
          131072.00               16.00                2.00
So, at this moment table size is 131072 and if again check the size of users tablespace then it :
SYS@orcl> /
TABLESPACE_NAME                MB_FREE
USERS                                 280100864 (280166400 – 65536)
Now, table T has been allocated space for 131072 bytes, but how much actual data it is carrying (data size of table) :
So, I will : (simple method by avg_row_len column, after analyze table)
select table_name,num_rows,avg_row_len, (num_rows*avg_row_len) "Data in Table" from user_tables where table_name='T';
TABLE_NAME                       NUM_ROWS AVG_ROW_LEN Data in Table
T                                       0           0             0
Now I will insert a row in the table :
insert into t values('Girish',40);
TABLE_NAME                       NUM_ROWS AVG_ROW_LEN Data in Table
T                                       0           0             0
analyze table t compute statistics;
TABLE_NAME                       NUM_ROWS AVG_ROW_LEN Data in Table
T                                       1          13            13
insert into t values('Alok',42);
TABLE_NAME                       NUM_ROWS AVG_ROW_LEN Data in Table
T                                       1          13            13
analyze table t compute statistics;
TABLE_NAME                       NUM_ROWS AVG_ROW_LEN Data in Table
T                                       2          12            24
Commit;
Ok.  At this moment it is having 24 bytes data.  Now I am going to delete 1 row :
delete from t where name='Girish';
TABLE_NAME                       NUM_ROWS AVG_ROW_LEN Data in Table
T                                       2          12            24
TABLE_NAME                       NUM_ROWS AVG_ROW_LEN Data in Table
T                                       1          11            11
Now I wish to gain 24-11=13 bytes to tablespace.
SCOTT@orcl> select bytes,blocks,extents from user_segments where segment_name='T';
     BYTES     BLOCKS    EXTENTS
    131072         16          2
Alter table t move;
SCOTT@orcl> select bytes,blocks,extents from user_segments where segment_name='T';
     BYTES     BLOCKS    EXTENTS
     65536          8          1
Means, it deallocated / released all extent/space and reached upto last shrinkable space.  Since I don’t have any associated index with this table; otherwise I will have to rebuild them.HTH
Girish Sharma