Nologging生效的时间

Nologging到底何时才能生效?

link:

http://www.itpub.net/242761.html

http://www.itpub.net/239905.html

?

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.?????????????????????????????????????????