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。
1 创建实验环境
1.1 创建ASSM的表空间
SQL> set serveroutput on
SQL> create tablespace ASSM datafile '/oradata/ltest/assm.dbf' size 10m autoextend on SEGMENT SPACE MANAGEMENT AUTO;
Tablespace created
SQL> select tablespace_name,
2 block_size,
3 extent_management,
4 allocation_type,
5 segment_space_management
6 from dba_tablespaces
7 where tablespace_name = 'ASSM';
TABLESPACE_NAME BLOCK_SIZE EXTENT_MANAGEMENT ALLOCATION_TYPE SEGMENT_SPACE_MANAGEMENT
--------------- ---------- ----------------- --------------- ------------------------
ASSM 8192 LOCAL SYSTEM AUTO
1.2 建表
SQL> create table my_objects tablespace assm as select * from all_objects;
Table created
SQL> select count(*) from my_objects;
COUNT(*)
----------
49903
2 实验前的信息
SQL> exec show_space('MY_OBJECTS');
Total Blocks ..........................768
Total Bytes ..........................6291456
Total MBytes ..........................6
Unused Blocks ..........................62
Unused Bytes ..........................507904
Unused KBytes ..........................496
Last Used Ext FileId....................7
Last Used Ext BlockId...................649
Last Used Block.........................66
The segment is analyzed below
FS1 Blocks (0-25) ....................0
FS2 Blocks (25-50) ....................0
FS3 Blocks (50-75) ....................0
FS4 Blocks (75-100) ....................0
Unformatted Blocks ....................0
Full Blocks ....................686
PL/SQL procedure successfully completed
3 删除后的信息
然后我们随机地从table MY_OBJECTS中删除一部分数据：
SQL> delete from my_objects where object_name like '%C%';
17674 rows deleted
SQL> delete from my_objects where object_name like '%U%';
4687 rows deleted
SQL> delete from my_objects where object_name like '%A%';
7010 rows deleted
SQL> exec show_space('MY_OBJECTS');
Total Blocks ..........................768
Total Bytes ..........................6291456
Total MBytes ..........................6
Unused Blocks ..........................62
Unused Bytes ..........................507904
Unused KBytes ..........................496
Last Used Ext FileId....................7
Last Used Ext BlockId...................649
Last Used Block.........................66
The segment is analyzed below
FS1 Blocks (0-25) ....................0
FS2 Blocks (25-50) ....................212
FS3 Blocks (50-75) ....................181
FS4 Blocks (75-100) ....................245
Unformatted Blocks ....................0
Full Blocks ....................48
PL/SQL procedure successfully completed
这里，table my_objects的HWM下有706(768 - 62)个block，其中，free space为25-50%的block有205个，free space为50-75%的block有180个，free space为75-100%的block有229个，full space的block只有45个，这种情况下，我们需要对这个table的现有数据行进行重组。
4 shink操作
要使用assm上的shink，首先我们需要使该表支持行移动，可以用这样的命令来完成：
SQL> alter table my_objects enable row movement;
Table altered
现在，就可以来降低my_objects的HWM，回收空间了，使用命令：
SQL> alter table my_objects shrink space;
Table altered
SQL> exec show_space('MY_OBJECTS');
Total Blocks ..........................280
Total Bytes ..........................2293760
Total MBytes ..........................2.1875
Unused Blocks ..........................5
Unused Bytes ..........................40960
Unused KBytes ..........................40
Last Used Ext FileId....................7
Last Used Ext BlockId...................265
Last Used Block.........................19
The segment is analyzed below
FS1 Blocks (0-25) ....................0
FS2 Blocks (25-50) ....................1
FS3 Blocks (50-75) ....................1
FS4 Blocks (75-100) ....................0
Unformatted Blocks ....................0
Full Blocks ....................259
PL/SQL procedure successfully completed
在执行玩shrink命令后，此时表my_objects的HWM现在降到了276(280 - 5 + 1)的位置，而且HWM下的block的空间使用状况，full space的block有259个，free space 为25-50% 和50-75% Block只有1个。
5 shrink space原理剖析
5.1 实验环境
SQL> create table TEST_HWM (id int ,name char(2000)) tablespace ASSM;
Table created
SQL> insert into TEST_HWM values (1, 'aa');
1 row inserted
SQL> insert into TEST_HWM values (2, 'bb');
1 row inserted
SQL> insert into TEST_HWM values (3, 'cc');
1 row inserted
SQL> insert into TEST_HWM values (4, 'ds');
1 row inserted
SQL> insert into TEST_HWM values (5, 'dss');
1 row inserted
SQL> insert into TEST_HWM values (6, 'dss');
1 row inserted
SQL> insert into TEST_HWM values (7, 'ess');
1 row inserted
SQL> insert into TEST_HWM values (8, 'es');
1 row inserted
SQL> insert into TEST_HWM values (9, 'es');
1 row inserted
SQL> insert into TEST_HWM values (10, 'es');
1 row inserted
5.2 删除前rowid状态
SQL> select id,
2 name,
3 rowid,
4 dbms_rowid.rowid_object(rowid) object_id,
5 dbms_rowid.rowid_relative_fno(rowid) file_id,
6 dbms_rowid.rowid_block_number(rowid) block_id,
7 dbms_rowid.rowid_row_number(rowid) num
8 from test_hwm;
ID NAME ROWID OBJECT_ID FILE_ID BLOCK_ID NUM
------ ----- ------------------ ---------- ---------- ---------- ----------
1 aa AAANAqAAHAAAAElAAA 53290 7 293 0
2 bb AAANAqAAHAAAAElAAB 53290 7 293 1
3 cc AAANAqAAHAAAAElAAC 53290 7 293 2
4 ds AAANAqAAHAAAAEmAAA 53290 7 294 0
5 dss AAANAqAAHAAAAEmAAB 53290 7 294 1
6 dss AAANAqAAHAAAAEmAAC 53290 7 294 2
7 ess AAANAqAAHAAAAEnAAA 53290 7 295 0
8 es AAANAqAAHAAAAEnAAB 53290 7 295 1
9 es AAANAqAAHAAAAEnAAC 53290 7 295 2
10 es AAANAqAAHAAAAEoAAA 53290 7 296 0
10 rows selected
5.3 删除后rowid状态
然后从table test_hwm中删除一些数据：
SQL> delete from TEST_HWM where id = 2;
1 row deleted
SQL> delete from TEST_HWM where id = 3;
1 row deleted
SQL> delete from TEST_HWM where id = 4;
1 row deleted
SQL> delete from TEST_HWM where id = 7;
1 row deleted
SQL> delete from TEST_HWM where id = 8;
1 row deleted
SQL> select id,
2 name,
3 rowid,
4 dbms_rowid.rowid_object(rowid) object_id,
5 dbms_rowid.rowid_relative_fno(rowid) file_id,
6 dbms_rowid.rowid_block_number(rowid) block_id,
7 dbms_rowid.rowid_row_number(rowid) num
8 from test_hwm;
ID NAME ROWID OBJECT_ID FILE_ID BLOCK_ID NUM
------ ----- ------------------ ---------- ---------- ---------- ----------
1 aa AAANAqAAHAAAAElAAA 53290 7 293 0
5 dss AAANAqAAHAAAAEmAAB 53290 7 294 1
6 dss AAANAqAAHAAAAEmAAC 53290 7 294 2
9 es AAANAqAAHAAAAEnAAC 53290 7 295 2
10 es AAANAqAAHAAAAEoAAA 53290 7 296 0
从以上的信息，可知表test_hwm中，剩下的数据是分布在AAAAEl，AAAAEm，AAAAEn，AAAAEo这样四个连续的block中。
SQL> exec show_space('TEST_HWM');
Total Blocks ..........................8
Total Bytes ..........................65536
Total MBytes ..........................0.0625
Unused Blocks ..........................0
Unused Bytes ..........................0
Unused KBytes ..........................0
Last Used Ext FileId....................7
Last Used Ext BlockId...................289
Last Used Block.........................8
The segment is analyzed below
FS1 Blocks (0-25) ....................0
FS2 Blocks (25-50) ....................1
FS3 Blocks (50-75) ....................3
FS4 Blocks (75-100) ....................1
Unformatted Blocks ....................0
Full Blocks ....................0
PL/SQL procedure successfully completed
通过show_space_assm我们可以看到目前这四个block的空间使用状况，AAAAEl，AAAAEn，AAAAEo上各有一行数据，可以猜测free space为50-75%的3个block是这三个block，那么free space为25-50%的1个block就是AAAAEm了，剩下free space为 75-100% 的3个block，是HWM下已格式化的尚未使用的block。
5.4 shrink后rowid状态
SQL> alter table my_objects enable row movement;
Table altered
SQL> alter table my_objects shrink space;
Table altered
SQL> select id,
2 name,
3 rowid,
4 dbms_rowid.rowid_object(rowid) object_id,
5 dbms_rowid.rowid_relative_fno(rowid) file_id,
6 dbms_rowid.rowid_block_number(rowid) block_id,
7 dbms_rowid.rowid_row_number(rowid) num
8 from test_hwm;
ID NAME ROWID OBJECT_ID FILE_ID BLOCK_ID NUM
------ ----- ------------------ ---------- ---------- ---------- ----------
9 es AAANAqAAHAAAAEkAAA 53290 7 292 0
10 es AAANAqAAHAAAAEkAAB 53290 7 292 1
1 aa AAANAqAAHAAAAElAAA 53290 7 293 0
5 dss AAANAqAAHAAAAEmAAB 53290 7 294 1
6 dss AAANAqAAHAAAAEmAAC 53290 7 294 2
当执行了shrink操作后，可以发现shrink操作与move不太一样。在move操作的时候，所有行的rowid都发生了变化，table所位于的block的区域也发生了变化，但是所有行物理存储的顺序都没有发生变化，所以我们得到的结论是，oracle以block为单位，进行了block间的数据copy。而在shrink后，部分行数据的rowid发生了变化，同时，部分行数据的物理存储的顺序也发生了变化，而table所位于的block的区域却没有变化（ID为1，5，6的rowid没有发生变化，ID为9，10两行数据，原来在AAAAEn，AAAAEo上都移到AAAAEk上）。以上说明，shrink只移动了table其中一部分的行数据，来完成释放空间，而且，这个过程是在table当前所使用的block中完成的。
6 shrink的注意点
1. move时产生的日志比shrink时少．参看http://blog.csdn.net/huang_xw/article/details/7016365
2. shrink在移动行数据时，也一起维护了index上相应行的数据rowid的信息，当然shrink过程中用来维护index的成本也会比较高。而表move后index的状态是UNUSABLE的,需要进行rebuild。参见http://blog.csdn.net/huang_xw/article/details/7016415
3. oracle是从后向前移动行数据，那么，shrink的操作就不会像move一样，shrink不需要使用额外的空闲空间。