delete 刪除資料 全表掃描還是掃描所有塊的測試

SQL> select count(distinct b) from
  2  (select dbms_rowid.rowid_block_number(rowid) b from t);

COUNT(DISTINCTB)
----------------
              80

 

SQL> select object_id,object_name from t where object_id=6318;

 OBJECT_ID
----------
OBJECT_NAME
--------------------------------------------------------------------------------

      6318
T2

 

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=9 Card=1 Bytes=19)
   1    0   TABLE ACCESS (FULL) OF 'T' (Cost=9 Card=1 Bytes=19)

 

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         80  consistent gets
          0  physical reads
          0  redo size
        443  bytes sent via SQL*Net to client
        503  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> delete from t where rownum<3000;    --刪除3000條記錄  總共6174條記錄

2999 rows deleted.

sql>commit

 

SQL> select object_id,object_name from t where object_id=6318;

 OBJECT_ID
----------
OBJECT_NAME
--------------------------------------------------------------------------------

      6318
T2

 

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=9 Card=1 Bytes=19)
   1    0   TABLE ACCESS (FULL) OF 'T' (Cost=9 Card=1 Bytes=19)

 

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         80  consistent gets    --刪除後全表掃描還是和原來一樣大
          0  physical reads
          0  redo size
        443  bytes sent via SQL*Net to client
        503  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> alter tablespace tools read write;

Tablespace altered.

SQL> alter table t move tablespace tools;   把表t移到另外個表空間tools   重新組織塊

Table altered.

SQL> select count(distinct b) from
  2  (select dbms_rowid.rowid_block_number(rowid) b from t);

COUNT(DISTINCTB)
----------------
              40

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT ptimizer=CHOOSE
   1    0   SORT (GROUP BY)
   2    1     TABLE ACCESS (FULL) OF 'T'

 

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         44  consistent gets   --一致性讀降低
         40  physical reads
         60  redo size
        387  bytes sent via SQL*Net to client
        503  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          1  rows processed