從10g開始,oracle開始提供Shrink的命令,假如我們的表空間中支援自動段空間管理 (ASSM),就可以使

用這個特性縮小段,即降低HWM.這裡需要強調一點,10g的這個新特性,僅對ASSM表空間有效,否則會報

ORA-10635: Invalid segment or tablespace type.

在這裡,我們來討論如和對一個ASSM的segment回收浪費的空間.

同樣,我們用系統檢視all_objects來在tablespace ASSM上建立測試表my_objects,這一小節的內容,實驗環

境為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 然後我們隨機地從table MY_OBJECTS中刪除一部分資料:

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 現在我們使用show_space和show_space_assm來看看my_objects的資料儲存狀況:

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 過程已成功完成.

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

PL/SQL 過程已成功完成. 這裡,table my_objects的HWM下有679個block,其中,free space為25-

50%的block有205個,free space為50-75%的block有180個,free space為75-100%的block有229個,full

space的block只有45個,這種情況下,我們需要對這個table的現有資料行進行重組.

要使用assm上的shink,首先我們需要使該表支援行移動,可以用這樣的命令來完成:

alter table my_objects enable row movement;

現在,就可以來降低my_objects的HWM,回收空間了,使用命令:

alter table bookings shrink space;

我們具體的看一下實驗的結果:

SQL> alter table my_objects enable row movement;

表已更改.

SQL> alter table my_objects shrink space;

表已更改.

SQL> exec show_space('MY_OBJECTS','DLINGER');

Total Blocks............................265

Total Bytes.............................2170880

Unused Blocks...........................2

Unused Bytes............................16384

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

Last Used Ext BlockId...................308

Last Used Block.........................3

PL/SQL 過程已成功完成.

SQL> exec show_space_assm('MY_OBJECTS','DLINGER');

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

free space 25-50% Blocks:...............1

free space 50-75% Blocks:...............0

free space 75-100% Blocks:..............0

Full Blocks:............................249

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

PL/SQL 過程已成功完成. 在執行玩shrink命令後,我們可以看到,table my_objects的HWM現在降到了

264的位置,而且HWM下的block的空間使用狀況,full space的block有249個,free space 為25-50%

Block只有1個.

我們接下來討論一下shrink的實現機制,我們同樣使用討論move機制的那個實驗來觀察.

SQL> create table TEST_HWM (id int ,name char(2000)) tablespace ASSM;

Table created 往table test_hwm中插入如下的資料:

insert into TEST_HWM values (1,'aa');

insert into TEST_HWM values (2,'bb');

insert into TEST_HWM values (2,'cc');

insert into TEST_HWM values (3,'dd');

insert into TEST_HWM values (4,'ds');

insert into TEST_HWM values (5,'dss');

insert into TEST_HWM values (6,'dss');

insert into TEST_HWM values (7,'ess');

insert into TEST_HWM values (8,'es');

insert into TEST_HWM values (9,'es');

insert into TEST_HWM values (10,'es');

我們來看看這個table的rowid和block的ID和資訊:

SQL> select rowid , id,name from TEST_HWM;

ROWID ID NAME

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

AAANhqAAGAAAAFHAAA 1 aa

AAANhqAAGAAAAFHAAB 2 bb

AAANhqAAGAAAAFHAAC 2 cc

AAANhqAAGAAAAFIAAA 3 dd

AAANhqAAGAAAAFIAAB 4 ds

AAANhqAAGAAAAFIAAC 5 dss

AAANhqAAGAAAAFJAAA 6 dss

AAANhqAAGAAAAFJAAB 7 ess

AAANhqAAGAAAAFJAAC 8 es

AAANhqAAGAAAAFKAAA 9 es

AAANhqAAGAAAAFKAAB 10 es

11 rows selected

SQL> select EXTENT_ID,FILE_ID,RELATIVE_FNO,BLOCK_ID,BLOCKS

2 from dba_extents where segment_name='TEST_HWM' ;

EXTENT_ID FILE_ID RELATIVE_FNO BLOCK_ID BLOCKS

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

0 6 6 324 5

1 6 6 329 5 然後從table test_hwm中刪除一些資料:

delete from TEST_HWM where id = 2; delete from TEST_HWM where id = 4; delete from TEST_HWM

where id = 3; delete from TEST_HWM where id = 7; delete from TEST_HWM where id = 8;

觀察table test_hwm的rowid和blockid的資訊:

SQL> select rowid , id,name from TEST_HWM;

ROWID ID NAME

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

AAANhqAAGAAAAFHAAA 1 aa

AAANhqAAGAAAAFIAAC 5 dss

AAANhqAAGAAAAFJAAA 6 dss

AAANhqAAGAAAAFKAAA 9 es

AAANhqAAGAAAAFKAAB 10 es

SQL> select EXTENT_ID,FILE_ID,RELATIVE_FNO,BLOCK_ID,BLOCKS

2 from dba_extents where segment_name='TEST_HWM' ;

EXTENT_ID FILE_ID RELATIVE_FNO BLOCK_ID BLOCKS

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

0 6 6 324 5

1 6 6 329 5 從以上的資訊,我們可以看到,在table test_hwm中,剩下的資料是分佈在

AAAAFH,AAAAFI,AAAAFJ,AAAAFK這樣四個連續的block中.

SQL> exec show_space_assm('TEST_HWM','DLINGER');

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

free space 25-50% Blocks:...............1

free space 50-75% Blocks:...............3

free space 75-100% Blocks:..............3

Full Blocks:............................0

Unformatted blocks:.....................0 透過show_space_assm我們可以看到目前這四個block的空間使用狀況

,AAAAFH,AAAAFI,AAAAFJ上各有一行資料,我們猜測free space為50-75%的3個block是這三個

block,那麼free space為25-50%的1個block就是AAAAFK了,剩下free space為 75-100% 的3個block,是

HWM下已格式化的尚未使用的block.(關於assm下hwm的移動我們前面已經詳細地討論過了,在extent不

大於於16個block時,是以一個extent為單位來移動的)

然後,我們對table my_objects執行shtink的操作:

SQL> alter table test_hwm enable row movement;

Table altered

SQL> alter table test_hwm shrink space;

Table altered

SQL> select rowid ,id,name from TEST_HWM;

ROWID ID NAME

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

AAANhqAAGAAAAFHAAA 1 aa

AAANhqAAGAAAAFHAAB 10 es

AAANhqAAGAAAAFHAAD 9 es

AAANhqAAGAAAAFIAAC 5 dss

AAANhqAAGAAAAFJAAA 6 dss

SQL> select EXTENT_ID,FILE_ID,RELATIVE_FNO,BLOCK_ID,BLOCKS

2 from dba_extents where segment_name='TEST_HWM' ;

EXTENT_ID FILE_ID RELATIVE_FNO BLOCK_ID BLOCKS

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

0 6 6 324 5

1 6 6 329 5 當執行了shrink操作後,有意思的現象出現了.我們來看看oracle是如何移動行資料的,這裡的情

況和move已經不太一樣了.我們知道,在move操作的時候,所有行的rowid都發生了變化,table所位於的

block的區域也發生了變化,但是所有行物理儲存的順序都沒有發生變化,所以我們得到的結論是,oracle以

block為單位,進行了block間的資料copy.那麼shrink後,我們發現,部分行資料的rowid發生了變化,同時

,部分行資料的物理儲存的順序也發生了變化,而table所位於的block的區域卻沒有變化,這就說明

,shrink只移動了table其中一部分的行資料,來完成釋放空間,而且,這個過程是在table當前所使用的

block中完成的.

那麼Oracle具體移動行資料的過程是怎樣的呢我們根據這樣的實驗結果,可以來猜測一下:

Oracle是以行為單位來移動資料的.Oracle從當前table儲存的最後一行資料開始移動,從當前table最先使用

的block開始搜尋空間,所以,shrink之前,rownum=10的那行資料(10,es),被移動到block AAAAFH上

,寫到(1,aa)這行資料的後面,所以(10,es)的rownum和rowid同時發生改變.然後是(9,es)這行資料

,重複上述過程.這是oracle從後向前移動行資料的大致遵循的規則,那麼具體移動行資料的的演算法是比較復

雜的,包括向ASSM的table中insert資料使用block的順序的演算法也是比較複雜的,大家有興趣的可以自己來研

究,在這裡我們不多做討論.

我們還可以在shrink table的同時shrink這個table上的index:

alter table my_objects shrink space cascade;

同樣地,這個操作只有當table上的index也是ASSM時,才能使用.

關於日誌的問題,我們對比了同樣資料量和分佈狀況的兩張table,在move和shrink下生成的redo

size(table上沒有index的情況下):

SQL> select tablespace_name,SEGMENT_SPACE_MANAGEMENT from dba_tablespaces

2 where tablespace_name in('ASSM','HWM');

TABLESPACE_NAME SEGMENT_SPACE_MANAGEMENT

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

ASSM AUTO

HWM MANUAL

SQL> create table my_objects tablespace ASSM as select * from all_objects where rownum create table my_objects1 tablespace HWM as select * from all_objects where rownum select bytes/1024/1024 from user_segments where segment_name = 'MY_OBJECTS';

BYTES/1024/1024

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

2.1875

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

7278 rows deleted

SQL> delete from my_objects1 where object_name like '%C%';

7278 rows deleted

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

2732 rows deleted

SQL> delete from my_objects1 where object_name like '%U%';

2732 rows deleted

SQL> commit;

Commit complete

SQL> alter table my_objects enable row movement;

Table altered

SQL> select value from v$mystat, v$statname

2 where v$mystat.statistic# = v$statname.statistic#

3 and v$statname.name = 'redo size';

VALUE

----------

27808792

SQL> alter table my_objects shrink space;

Table altered

SQL> select value from v$mystat, v$statname

2 where v$mystat.statistic# = v$statname.statistic#

3 and v$statname.name = 'redo size';

VALUE

----------

32579712

SQL> alter table my_objects1 move;

Table altered

SQL> select value from v$mystat, v$statname

2 where v$mystat.statistic# = v$statname.statistic#

3 and v$statname.name = 'redo size';

VALUE

----------

32676784 對於table my_objects,進行shrink,產生了32579712 – 27808792=4770920,約4.5M的redo

;對table my_objects1進行move,產生了32676784-32579712= 97072,約95K的redo size.那麼,與

move比較起來,shrink的日誌寫要大得多.

Shrink的幾點問題:

a. shrink後index是否需要rebuild:

因為shrink的操作也會改變行資料的rowid,那麼,如果table上有index時,shrink table後index會不會變為

UNUSABLE呢我們來看這樣的實驗,同樣構建my_objects的測試表:

create table my_objects tablespace ASSM as select * from all_objects where rownum<20000;

create index i_my_objects _disibledevent= 'MY_OBJECTS';

OBJECT_ID

----------

55422 那麼,當table在進行shrink時,我們對table是可以進行DML操作的.

c. shrink對空間的要求

我們在前面討論了shrink的資料的移動機制,既然oracle是從後向前移動行資料,那麼,shrink的操作就不會像

move一樣,shrink不需要使用額外的空閒空間. 2008-7-29 9:56:59

assm:Oracle 10g的自動段空間管理

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