深入探讨Oracle数据库10g的Shrink机制

从10g开始，oracle开始提供Shrink的命令，假如我们的表空间中支持自动段空间管理 (ASSM),就可以使用这个特性缩小段，即降低HWM。这里需要强调一点，10g的这个新特性，仅对ASSM表空间有效，否则会报 ORA-10635: Invalid segment or tablespace type。

在这里，我们来讨论如和对一个ASSM的segment回收浪费的空间。

同样，我们用系统视图all_objects来在tablespace ASSM上创建测试表my_objects，这一小节的内容，实验环境为oracle10.1.0.2：

SQL> select * from v$version;

BANNER

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

Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - Prod

PL/SQL Release 10.1.0.2.0 - Production

CORE 10.1.0.2.0 Production

TNS for 32-bit Windows: Version 10.1.0.2.0 - Production

NLSRTL Version 10.1.0.2.0 – Production

SQL> select TABLESPACE_NAME,BLOCK_SIZE,EXTENT_MANAGEMENT,

2 ALLOCATION_TYPE, SEGMENT_SPACE_MANAGEMENT

3 from dba_tablespaces where TABLESPACE_NAME = ASSM;

TABLESPACE_NAME BLOCK_SIZE EXTENT_MANAGEMENT ALLOCATION_TYPE SEGMENT_SPACE_MANAGEMENT

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

ASSM 8192 LOCAL UNIFORM AUTO

SQL> create table my_objects tablespace assm

2 as select * from all_objects;

Table created

然后我们随机地从table MY_OBJECTS中删除一部分数据：

SQL> select count(*) from my_objects;

COUNT(*)

----------

47828

SQL> delete from my_objects where object_name like %C%;

16950 rows deleted

SQL> delete from my_objects where object_name like %U%;

4503 rows deleted

SQL> delete from my_objects where object_name like %A%;

6739 rows deleted

现在我们使用show_space和show_space_assm来看看my_objects的数据存储状况：

SQL> exec show_space(MY_OBJECTS,DLINGER);

Total Blocks............................680

Total Bytes.............................5570560

Unused Blocks...........................1

Unused Bytes............................8192

Last Used Ext FileId....................6

Last Used Ext BlockId...................793

Last Used Block.........................4

PL/SQL 过程已成功完成。

SQL> exec show_space_assm(MY_OBJECTS,DLINGER);

free space 0-25% Blocks:................0

free space 25-50% Blocks:...............205

free space 50-75% Blocks:...............180

free space 75-100% Blocks:..............229

Full Blocks:............................45

Unformatted blocks:.....................0