undo只是逻辑还原

undo只是逻辑还原数据库而不是物理的还原：

测试：

1.新建一个空表

create table t1

(

sid int not null primary key,

sname varchar2(10)

)tablespace test;

create table t2

(

sid int not null primary key,

sname varchar2(10)

)tablespace test;

select * from t2;

2.做全表扫描并查看I/O数量

set autotrace traceonly statistics;

select * from t2;

统计信息

----------------------------------------------------------

0 recursive calls

0 db block gets

3 consistent gets

0 physical reads

0 redo size

399 bytes sent via SQL*Net to client

509 bytes received via SQL*Net from client

1 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

0 rows processed

set autotrace off;

--循环导入数据

declare

maxrecords constant int:=20000;

i int :=1;

begin

for i in 1..maxrecords loop

insert into t1 values(i,'ocpyang');

end loop;

dbms_output.put_line(' 成功录入数据！ ');

commit;

end;

/

insert into t2 select * from t1;

rollback;



set autotrace traceonly statistics;

select * from t2;

统计信息

----------------------------------------------------------

0 recursive calls

0 db block gets

53 consistent gets

0 physical reads

0 redo size

399 bytes sent via SQL*Net to client

509 bytes received via SQL*Net from client

1 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

0 rows processed

虽然数据rollback但是insert的操作还是导致表的高水位线，这些数据块并

没有因为rollback而回滚.

验证了数据库并没有物理完全恢复，仅仅是逻辑恢复！