Oracle9i之前，表数据的整理是通过 alter table XXX move [tablespace XX]进行的。如果表非常大，IO又不快的时候，move的过程可能相当漫长，不能算是HA特性。因此在HA的在线维护中，基本不会利用move来重组大型表，而且move后的表需要重建索引。而在9i引入的dbms_redefinition。该过程的内部原理其实就是采用了MV的机制，类似在原表建立一个prebuilt的MV然后一直增量刷新到数据差别最小。Oracle在很小的停顿中，完成最后一点的增量同步，达到完全同步后，把原表与新表换个名字，由于换名操作仅仅是数据字典，所以最终切换时间非常短。
--首先建立一个用来测试外键约束的表test_refedSQL> create table test_refed(id number primary key);Table created.SQL> insert into test_refed select rownum from dba_objects where rownum<10001;10000 rows created.SQL> commit;Commit complete.--创建我们准备修改表定义的表test，未分区SQL> create table test(id number,fid number);Table created.SQL> insert into test select rownum,rownum from dba_objects where rownum<1001;1000 rows created.SQL> commit;Commit complete.--添加主键约束以及外键约束，并在外键上建立索引SQL> alter table test add constraint pk_test primary key(id);Table altered.SQL> alter table test add constraint fk_test_refed_id foreign key(fid) references test_refed(id);Table altered.SQL> create index idx_test_fid on test(fid);Index created.--创建基于test表的触发器SQL> create trigger tr_test 2 before insert on test for each row3 begin4 null;5 end;6 /Trigger created.--准备工作：调用dbms_redefinition.can_redef_table来验证boylook.test是否可以在线重定义。一般情况没有主键等会报错。SQL> begin 2 dbms_redefinition.can_redef_table('BOYLOOK','TEST');3 end;4 /PL/SQL procedure successfully completed.--创建需要重新定义的过渡表inter_test，这是一个分区表，以后将把原表所有数据在线转移到该表当中。注意到，该表比原表test还多一个字段cSQL> create table inter_test(id number,fid number,c number)2 partition by range(id)3 (partition p1 values less than(400),4 partition p2 values less than(800),5 partition p3 values less than(maxvalue));Table created.SQL> alter table inter_test add constraint pk_inter_test primary key(id);Table altered.--执行在线重定义注：调用这个存储过程需要create/alter/drop/lock/select any table权限SQL> exec dbms_redefinition.start_redef_table('BOYLOOK','TEST','INTER_TEST','id id,fid fid,0 c');PL/SQL procedure successfully completed.--验证数据是否刷了过去SQL> select count(*) from inter_test;COUNT(*)----------1000--继续对原表test进行操作SQL> insert into test select rownum+1000,rownum+1000 from dba_objects where rownum <=24;24 rows created.SQL> commit;Commit complete.SQL> select count(*) from test;COUNT(*)----------1024SQL> select count(*) from inter_test;COUNT(*)----------1000--执行表同步注：这一步不是必须的，但是对于比较大的表，中间运行增量同步有助于减少切换时间。SQL> exec dbms_redefinition.sync_interim_table('BOYLOOK','TEST','INTER_TEST');PL/SQL procedure successfully completed.--我们发现数据同步的过来SQL> select count(*) from inter_test;COUNT(*)----------1024--将原表test的约束，索引，触发器迁移过来
注：这里***要检查一下授权。检查test以前的权限，并给中间表inter_test赋予同样的权限。否则原表的权限不会转移到新表SQL> alter table inter_test add constraint fk_inter_refed_id foreign key(fid) references test_refed(id);Table altered.SQL> create index idx_inter_test_fid on inter_test(fid); Index created.SQL> create or replace trigger tr_inter_test2 before insert on inter_test for each row3 begin4 null;5 end;6 /Trigger created.--执行重定义完成的过程。Oracle完成了表test到表inter_test的换名工作，只是所有的约束，索引或触发器名称还是保持着原来表上面的名称SQL> exec dbms_redefinition.finish_redef_table('BOYLOOK','TEST','INTER_TEST');PL/SQL procedure successfully completed.SQL> desc test;Name Null? Type----------------------------------------- -------- ----------------------------ID NOT NULL NUMBERFID NUMBERC NUMBERSQL> desc inter_test;Name Null? Type----------------------------------------- -------- ----------------------------ID NOT NULL NUMBERFID NUMBERSQL> select table_name,partition_name from user_tab_partitions where table_name ='TEST';TABLE_NAME PARTITION_NAME------------------------------ ------------------------------TEST P1TEST P2TEST P3SQL> select table_name,constraint_name,status from user_constraints where table_name in('TEST','INTER_TEST');TABLE_NAME CONSTRAINT_NAME STATUS------------------------------ ------------------------------ --------INTER_TEST PK_TEST ENABLEDINTER_TEST FK_TEST_REFED_ID DISABLEDTEST PK_INTER_TEST ENABLEDTEST FK_INTER_REFED_ID ENABLEDSQL> select table_name,index_name from user_indexes where table_name in('TEST','INTER_TEST');TABLE_NAME INDEX_NAME------------------------------ ------------------------------TEST PK_INTER_TESTTEST IDX_INTER_TEST_FIDINTER_TEST PK_TESTINTER_TEST IDX_TEST_FIDSQL> select table_name,trigger_name from user_triggers where table_name in('TEST','INTER_TEST');TABLE_NAME TRIGGER_NAME------------------------------ ------------------------------TEST TR_INTER_TESTINTER_TEST TR_TESTSQL> select * from test where rownum <=10;ID FID C---------- ---------- ----------1 1 02 2 03 3 04 4 05 5 06 6 07 7 08 8 09 9 010 10 010 rows selected.--删除过渡表了，收工。这时也可以考虑修改索引，约束触发器的名称与原来保持一致SQL> drop table inter_test cascade constraints purge;Table dropped.
本文转自MIKE老毕 51CTO博客，原文链接：http://blog.51cto.com/boylook/1298615，如需转载请自行联系原作者
