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
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
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-