Oracle 11gR2新建空表不分配Segment



一、引言：

ORACLE10G，新建空表以后立刻就分配Segment，
Oracle11gR2，新建空表后没有立即分配Segment。这就是11GR2的新特性，延迟段创建，就是说从11GR2开始默认创建的表不会立即分配segment，不会占用磁盘空间，当第一条数据insert时才会分配空间。

二、实验模拟：

SQL>
select
*
from v$version; 

 

BANNER

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

Oracle
Database 11g Enterprise Edition Release
11.2.0.1.0
- Production

PL/SQL Release
11.2.0.1.0
- Production

CORE   
11.2.0.1.0   
Production

TNS
for Linux: Version
11.2.0.1.0
- Production

NLSRTL Version
11.2.0.1.0
- Production

 

SQL>
create
table
testnew(id int
primary
key,name
varchar2(10)); 

 

Table created

 

SQL>
create
table
testnew_IME(idint
primary
key,name
varchar2(10))
segment creationimmediate;

 

Table created

 

SQL>
create
table
testnew_def(idint
primary
key,name
varchar2(10))
segment creationdeferred; 

 

Table created

 

SQL>
select segment_name
from user_segments
where segment_name
like
'TESTNEW%'; 

 

SEGMENT_NAME

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

TESTNEW_IME

 

SQL>
select INDEX_NAME,TABLE_OWNER
from USER_indexes
where table_name='TESTNEW'; 

 

INDEX_NAME                    
TABLE_OWNER

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

SYS_C0011192     
             JACK

 

SQL>
select INDEX_NAME,TABLE_OWNER
from USER_indexes
where table_name='TESTNEW_IME'; 

 

INDEX_NAME                    
TABLE_OWNER

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

SYS_C0011193                  
JACK

 

SQL>
select INDEX_NAME,TABLE_OWNER
from USER_indexes
where table_name='TESTNEW_DEF'; 

 

INDEX_NAME                    
TABLE_OWNER

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

SYS_C0011194                  
JACK

 

SQL>
select segment_name
from user_segments
where segment_name='SYS_C0011192'; 

 

SEGMENT_NAME

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

 

SQL>
select segment_name
from user_segments
where segment_name='SYS_C0011193';

 

SEGMENT_NAME

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

SYS_C0011193

 

SQL>
select segment_name
from user_segments
where segment_name='SYS_C0011194';

 

SEGMENT_NAME

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

 

SQL>
insert
into testnew
values(1,'anbob.com'); 

 

1 row inserted

 

SQL>
commit;

 

Commit complete

 

SQL>
select segment_name
from user_segments
where segment_name
like
'TESTNEW%'; 

 

SEGMENT_NAME

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

TESTNEW

TESTNEW_IME

 

SQL>
select INDEX_NAME,TABLE_OWNER
from USER_indexes
where table_name='TESTNEW'; 

 

INDEX_NAME                    
TABLE_OWNER

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

SYS_C0011192                  
JACK

 

SQL>
select segment_name
from user_segments
where segment_name='SYS_C0011192';

 

SEGMENT_NAME

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

SYS_C0011192

 

SQL>
truncate
table testnew; 

 

Table truncated

 

SQL>
select segment_name
from user_segments
where segment_name
like
'TESTNEW%'; 

 

SEGMENT_NAME

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

TESTNEW

TESTNEW_IME

 

SQL>
conn /as
sysdba

已连接。

SQL>
create
table testnew_def(id
int
primary
key,name
varchar2(10))
segment creation deferred; 

create
table testnew_def(id
int
primary
key,name
varchar2(10))
segment creation deferred

*

第
1
行出现错误:

ORA-14223:
此表不支持延迟创建段

---sys schema里不支持，exp也不会导出

注意：

11gR2默认是使用segmentcreation deferred建立，新建的无记录表不分配segment，当insert第一条记录时分配段空间，不会因truncate而回收，并且在sysschema里不支持，exp也不会导出。

关于这个主要还是跟deferred_segment_creation参数有关，在11gR2中该参数的值为true，说明当创建对象(如表)，初始没有数据，不会立即创建segment。

如果该参数设置为false，表明之后的创建的表，初始没有数据，会立即创建segment。

下面看一下它的效果：

SQL>
alter system
set deferred_segment_creation=false;

 

System altered

 

SQL>
create
table jack(x
int);

 

Table created

 

SQL>
select segment_name,segment_type,extents,blocks
from user_segments
where segment_name='JACK';

 

SEGMENT_NAME                                                                    SEGMENT_TYPE         
EXTENTS     BLOCKS

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

JACK                                                                            TABLE                      
1         
8

参考文章：http://www.cnblogs.com/Richardzhu/p/3627653.html