Data block information after deletion of data

Question

Hi , If I have deleted a row in a particular table then which meta data table in oracle will record that the no of bytes got deducted.
I have checked in this table for perticular table before deletion and after deletion but there is no difference in number...DBA_SEGMENTS
Edited by: 883279 on Jan 28, 2013 4:56 AM

Answer 1

883279 wrote:
Hi , If I have deleted a row in a particular table then which meta data table in oracle will record that the no of bytes got deducted.
I have checked in this table for perticular table before deletion and after deletion but there is no difference in number...DBA_SEGMENTS
Edited by: 883279 on Jan 28, 2013 4:56 AM When you delete a row in a table, oracle marks the block as free block and it does not actually frees the block. The subsequent insert statement uses this information and overwrites the block with new data.
cehck this out, I have a table
SQL> create table t
  2  as
  3  select *
  4    from all_objects;
Table created.Now i run this
SQL> set autotrace on
SQL>
SQL> select count(*) from t;
  COUNT(*)
    213321
Statistics
         28  recursive calls
          0  db block gets
       2960  consistent gets
       2879  physical reads
          0  redo size
        517  bytes sent via SQL*Net to client
        488  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> select count(*) from t;
  COUNT(*)
    213321
Statistics
          0  recursive calls
          0  db block gets
       2884  consistent gets
          0  physical reads
          0  redo size
        517  bytes sent via SQL*Net to client
        488  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedCheck the consistent gets in the second qyery its 2884.
Now i delete all the rows from table
SQL> delete from t;
213321 rows deleted.Now i again run the query
SQL> select count(*) from t;
  COUNT(*)
         0
Statistics
          0  recursive calls
          0  db block gets
       2884  consistent gets
        306  physical reads
          0  redo size
        514  bytes sent via SQL*Net to client
        488  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedYou can see the consistent gets is same 2884, so oracle goes through all the data blocks even after
we delete them.
Now i truncate the table
SQL> truncate table t;
Table truncated.
SQL> select count(*) from t;
  COUNT(*)
         0
Statistics
          1  recursive calls
          1  db block gets
          6  consistent gets
          0  physical reads
         52  redo size
        514  bytes sent via SQL*Net to client
        488  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedNow the HWM is reset and the blocks are released.