结论
1,alter table drop unused columns checkpoint适用于减少不必要的undo空间消耗 2,checkpoint选项可取值有： 不取值，默认在512条记录后发生检查点动作 大于表记录个数，表明在处理完所有表记录后发生检查点 小于表记录个数，表明在指定表记录后发生检查点 3，如果checkpoint选项后产生的检查点动作被中断，仅truncate table,drop table,alter table drop unused columns continue语句可以运行 4，alter table drop unused columns continue即恢复继续执行被中断的alter table drop unused columns checkpoint继续下去
测试
1,数据库版本 SQL> select * from v$version where rownum=1; BANNER ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 2，开启检查点信息到告警日志 SQL> show parameter checkpoint NAME TYPE VALUE ------------------------------------ ---------------------- ------------------------------ log_checkpoint_interval integer 0 log_checkpoint_timeout integer 1800 log_checkpoints_to_alert boolean FALSE SQL> alter system set log_checkpoints_to_alert=true; System altered. [oracle@seconary trace]$ 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,创建测试表并插入数据 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，指定测试表B列为不可用 SQL> alter table t_checkpoint set unused column b; Table altered. 5，可见指定checkpoint选项后会发生一个检查点动作 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,如果正在进行检查点动作，被中断，会如何呢？ 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 可见如果检查点进程被中断，不能进行alter table add操作，必须先要让检查点运行完成，即运行 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查询也不成 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用于继续执行被中断的检查点选项的删除列操作 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删除表不可用列系列二_ITPUB博客
