alter table drop unused columns checkpoint

結論

1,alter table drop unused columns checkpoint適用於減少不必要的undo空間消耗

2,checkpoint選項可取值有：
    不取值，預設在512條記錄後發生檢查點動作
    大於表記錄個數，表明在處理完所有表記錄後發生檢查點
    小於表記錄個數，表明在指定表記錄後發生檢查點

3，如果checkpoint選項後產生的檢查點動作被中斷，僅truncate table,drop table,alter table drop unused columns continue語句可以執行

4，alter table drop unused columns continue即恢復繼續執行被中斷的alter table drop unused columns checkpoint繼續下去    

測試

1,資料庫版本
SQL> select * from v$version where rownum=1;


BANNER
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production


2，開啟檢查點資訊到告警日誌
SQL> show parameter checkpoint


NAME                                 TYPE                   VALUE
------------------------------------ ---------------------- ------------------------------
log_checkpoint_interval              integer                0
log_checkpoint_timeout               integer                1800
log_checkpoints_to_alert             boolean                FALSE


SQL> alter system set log_checkpoints_to_alert=true;


System altered.


[oracle@seconary trace]$ tail -f alert_guowang.log 
  Current log# 2 seq# 356 mem# 0: /oracle/oradata/guowang/redo02.log
Fri Oct 16 08:18:16 2015
Thread 1 advanced to log sequence 357 (LGWR switch)
  Current log# 3 seq# 357 mem# 0: /oracle/oradata/guowang/redo03.log
Fri Oct 16 08:27:17 2015
ALTER SYSTEM SET log_checkpoints_to_alert=TRUE SCOPE=BOTH;
Fri Oct 16 08:28:44 2015
Beginning log switch checkpoint up to RBA [0x166.2.10], SCN: 11696352
Thread 1 advanced to log sequence 358 (LGWR switch)
  Current log# 1 seq# 358 mem# 0: /oracle/oradata/guowang/redo01.log


SQL> alter system checkpoint;


System altered.


Fri Oct 16 08:29:15 2015
Beginning global checkpoint up to RBA [0x166.1044.10], SCN: 11698731
Completed checkpoint up to RBA [0x166.1044.10], SCN: 11698731
Completed checkpoint up to RBA [0x166.2.10], SCN: 11696352


3,建立測試表並插入資料
SQL> conn scott/system
Connected.
SQL> create table t_checkpoint(a int,b int);


Table created.


SQL> insert into t_checkpoint select level,level from dual connect by level<=100000;


100000 rows created.


SQL> commit;


Commit complete.


4，指定測試表B列為不可用
SQL> alter table t_checkpoint set unused column b;


Table altered.


5，可見指定checkpoint選項後會發生一個檢查點動作


SQL> alter table t_checkpoint drop unused columns checkpoint 500;


Table altered.


Fri Oct 16 08:34:11 2015
Beginning log switch checkpoint up to RBA [0x167.2.10], SCN: 11720087
Thread 1 advanced to log sequence 359 (LGWR switch)
  Current log# 2 seq# 359 mem# 0: /oracle/oradata/guowang/redo02.log




6,如果正在進行檢查點動作，被中斷，會如何呢？
SQL> drop table t_checkpoint purge;


Table dropped.


SQL> create table t_checkpoint(a int,b int);


Table created.


SQL> insert into t_checkpoint select level,level from dual connect by level<=100000;


100000 rows created.


SQL> commit;


Commit complete.  


SQL> alter table t_checkpoint drop unused columns checkpoint;


alter table t_checkpoint drop unused columns checkpoint
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-01013: user requested cancel of current operation






SQL> SQL> 


SQL> select table_name,status from user_tables where table_name='T_CHECKPOINT';


TABLE_NAME                                                   STATUS
------------------------------------------------------------ ----------------
T_CHECKPOINT                                                 VALID


可見如果檢查點程式被中斷，不能進行alter table add操作，必須先要讓檢查點執行完成，即執行
  alter table drop columns continue
SQL> alter table t_checkpoint add c int;
alter table t_checkpoint add c int
            *
ERROR at line 1:
ORA-12986: columns in partially dropped state. Submit ALTER TABLE DROP COLUMNS
CONTINUE




SQL> 


SQL> host oerr ora 12986
12986, 00000, "columns in partially dropped state. Submit ALTER TABLE DROP COLUMNS CONTINUE"
// *Cause:  An attempt was made to access a table with columns in partially 
//          dropped state (i.e., drop column operation was interrupted).
// *Action: Submit ALTER TABLE DROP COLUMNS CONTINUE to complete the drop
//          column operation before accessing the table.


select查詢也不成
SQL> select count(*) from t_checkpoint;
select count(*) from t_checkpoint
                     *
ERROR at line 1:
ORA-12986: columns in partially dropped state. Submit ALTER TABLE DROP COLUMNS
CONTINUE


truncate操作是可以成功的
SQL> truncate table t_checkpoint;


Table truncated.




alter table drop columns continue用於繼續執行被中斷的檢查點選項的刪除列操作
SQL> alter table t_checkpoint drop columns continue;


Table altered.


SQL> alter table t_checkpoint add c int;


Table altered.

出自：

oracle 11g之alter table drop unused columns checkpoint刪除表不可用列系列二_ITPUB部落格