oracle中rollback操作产生redo信息

#进行update操作：
sys@ORCL> update hn.emp set sal=3000 where empno=7902;
sys@ORCL> select b.name,a.value
  2  from v$mystat a,v$statname b
  3  where a.statistic#=b.statistic# and b.name in ('redo size','undo change vector size');

NAME                                VALUE
------------------------------ ----------
redo size                             460
undo change vector size               160
#update操作产生了460大小redo信息，160大小undo信息

sys@ORCL> rollback;
Rollback complete.

sys@ORCL> select b.name,a.value
  2  from v$mystat a,v$statname b
  3  where a.statistic#=b.statistic# and b.name in ('redo size','undo change vector size');

NAME                                VALUE
------------------------------ ----------
redo size                             784
undo change vector size               160
结论1：
    rollback操作也产生redo 信息。

sys@ORCL> update hn.emp set sal=3000 where empno=7902;

1 row updated.

sys@ORCL> select b.name,a.value
  2  from v$mystat a,v$statname b
  3  where a.statistic#=b.statistic# and b.name in ('redo size','undo change vector size');

NAME                                VALUE
------------------------------ ----------
redo size                            1244
undo change vector size               320

sys@ORCL> commit;   
Commit complete.

sys@ORCL> select b.name,a.value
  2  from v$mystat a,v$statname b
  3  where a.statistic#=b.statistic# and b.name in ('redo size','undo change vector size');

NAME                                VALUE
------------------------------ ----------
redo size                            1384
undo change vector size               320

结论2：
    commit提交瞬间，也产生redo信息，是因为要讲最后的那些没有提交的redo信息写入到redo log file中
    select语句不产生redo和undo信息。