Nologging到底何时才能生效

转了一篇EYGLE的文章

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最初的问题是这个帖子:

http://www.itpub.net/showthread.php?threadid=239905

请大家仔细看那些测试的例子.

看了Tom的解释,始终觉得牵强.
开始以为可能是bug
经过观察和测试,终于发现了Nologging的秘密

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我们知道,Nologging只在很少情况下生效
通常,DML操作总是要生成redo的

这个我们不多说.

关于Nologging和append,一直存在很多误解.
经过一系列研究,终于发现了Nologging的真相.

我们来看一下测试:

1.Nologging的设置跟数据库的运行模式有关

a.数据库运行在非归档模式下:

SQL> archive log list;
Database log mode              No Archive Mode
Automatic archival             Enabled
Archive destination            /opt/oracle/oradata/hsjf/archive
Oldest online log sequence     155
Current log sequence           157

SQL> @redo
SQL> create table test as select * from dba_objects where 1=0;

Table created.

SQL> select * from redo_size;

     VALUE
----------
     63392

SQL>
SQL> insert into test select * from dba_objects;

10470 rows created.

SQL> select * from redo_size;

     VALUE
----------
   1150988

SQL>
SQL> insert /*+ append */ into test select * from dba_objects;

10470 rows created.

SQL> select * from redo_size;

     VALUE
----------
   1152368

SQL> select (1152368 -1150988) redo_append,(1150988 -63392) redo from dual;

REDO_APPEND       REDO
----------- ----------
       1380    1087596

SQL> drop table test;

Table dropped.
   

我们看到在Noarchivelog模式下,对于常规表的insert append只产生少量redo

注：redo_size是一个自定义视图：

create or replace view redo_size
as
select value
  from v$mystat, v$statname
where v$mystat.statistic# = v$statname.statistic#
   and v$statname.name = 'redo size';

b.在归档模式下

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area  235999908 bytes
Fixed Size                   451236 bytes
Variable Size             201326592 bytes
Database Buffers           33554432 bytes
Redo Buffers                 667648 bytes
Database mounted.

SQL> alter database archivelog;

Database altered.

SQL> alter database open;

Database altered.

SQL> @redo
SQL> create table test as select * from dba_objects where 1=0;

Table created.

SQL> select * from redo_size;

     VALUE
----------
     56288

SQL>
SQL> insert into test select * from dba_objects;

10470 rows created.

SQL> select * from redo_size;

     VALUE
----------
   1143948

SQL>
SQL> insert /*+ append */ into test select * from dba_objects;

10470 rows created.

SQL> select * from redo_size;

     VALUE
----------
   2227712

SQL> select (2227712 -1143948) redo_append,(1143948 -56288) redo from dual;

REDO_APPEND       REDO
----------- ----------
    1083764    1087660

SQL> drop table test;

Table dropped.
   

我们看到在归档模式下,对于常规表的insert append产生和insert同样的redo
此时的insert append实际上并不会有性能提高.
但是此时的append是生效了的

通过Logmnr分析日志得到以下结果:

SQL> select operation,count(*)
  2  from v$logmnr_contents
  3  group by operation;

OPERATION                          COUNT(*)
-------------------------------- ----------
COMMIT                                   17
DIRECT INSERT                         10470
INTERNAL                                 49
START                                    17
                                          1
    

我们注意到这里是DIRECT INSERT,而且是10470条记录,也就是每条记录都记录了redo.

2.对于Nologging的table的处理

a. 在归档模式下:

SQL> create table test nologging as select * from dba_objects where 1=0;

Table created.

SQL> select * from redo_size;

     VALUE
----------
   2270284

SQL>
SQL> insert into test select * from dba_objects;

10470 rows created.

SQL> select * from redo_size;

     VALUE
----------
   3357644

SQL>
SQL> insert /*+ append */ into test select * from dba_objects;

10470 rows created.

SQL> select * from redo_size;

     VALUE
----------
   3359024

SQL> select (3359024 -3357644) redo_append,(3357644 - 2270284) redo from dual;

REDO_APPEND       REDO
----------- ----------
       1380    1087360

SQL> drop table test;

Table dropped.
    

我们注意到,只有append才能减少redo

b.在非归档模式下:

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area  235999908 bytes
Fixed Size                   451236 bytes
Variable Size             201326592 bytes
Database Buffers           33554432 bytes
Redo Buffers                 667648 bytes
Database mounted.
SQL> alter database noarchivelog;

Database altered.

SQL> alter database open;

Database altered.

SQL> @redo
SQL> create table test nologging as select * from dba_objects where 1=0;

Table created.

SQL> select * from redo_size;

     VALUE
----------
     56580

SQL>
SQL> insert into test select * from dba_objects;

10470 rows created.

SQL> select * from redo_size;

     VALUE
----------
   1144148

SQL>
SQL> insert /*+ append */ into test select * from dba_objects;

10470 rows created.

SQL> select * from redo_size;

     VALUE
----------
   1145528

SQL> select (1145528 -1144148) redo_append,(1144148 -56580) redo from dual;

REDO_APPEND       REDO
----------- ----------
       1380    1087568

SQL>
    

同样只有append才能减少redo的生成.

这就是通常大家认识的情况.

-The End-