oracle 删除表中重复记录
表demo是重复拷贝自dba_objects，有88万左右，不重复的是27323，没有索引
方法一：delete from demo a where a.rowid <> (select max(rowid) from demo b where
                                                                      b.object_id=a.object_id);
            耗时：几个小时以上
方法二： delete from demo where rowid in
              (select rid from
                          (select rowid rid,row_number() over(partition by object_id order by rowid) rn
                             from demo)
               where rn <> 1 );
   耗时：30秒

方法三： create table demo2 as
              select object_id,owner... from
                       (select demo.*,row_number() over(partition by object_id order by rowid) rn from demo)
                         where rn = 1;
              truncate table demo; insert into demo select * from demo2; drop table demo2;
共耗时： 10秒，适合大数据量的情况，产生更少回滚量；