alter table move跟shrink space的区别

Bertha 。 2022-07-12 10:56 140阅读 0赞

author：skate

time2010-05-28

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

**alter table move跟shrink space的区别**

今天主要从两点说他们的区别：

1. 碎片的整理

2.空间的收缩

SQL> select \* from v$version;

BANNER  
\----------------------------------------------------------------  
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi  
PL/SQL Release 10.2.0.1.0 - Production  
CORE 10.2.0.1.0 Production

TNS for Linux: Version 10.2.0.1.0 - Production  
NLSRTL Version 10.2.0.1.0 - Production

SQL>

创建测试表  
SQL> create table test3 as  
  2     select rownum id,  
  3     dbms\_random.string('a', round(dbms\_random.value(0,10))) col1,  
  4     trunc(sysdate) - dbms\_random.value(1, 365\*2) col2  
  5     from dual connect by rownum<=10000;

Table created

SQL> select count(1) from test3;

COUNT(1)  
\----------  
     10000

查看表test3的blocks使用情况：

SQL> exec show\_space\_t('TEST3','auto','table','Y');

Total Blocks............................40  
Total Bytes.............................327680  
Unused Blocks...........................3  
Unused Bytes............................24576  
Last Used Ext FileId....................31  
Last Used Ext BlockId...................481921  
Last Used Block.........................5  
 \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*  
0% -- 25% free space blocks.............0  
0% -- 25% free space bytes..............0  
25% -- 50% free space blocks............0  
25% -- 50% free space bytes.............0  
50% -- 75% free space blocks............0  
50% -- 75% free space bytes.............0  
75% -- 100% free space blocks...........0  
75% -- 100% free space bytes............0  
Unused Blocks...........................0  
Unused Bytes............................0  
Total Blocks............................32  
Total bytes.............................262144

PL/SQL procedure successfully completed

制造碎片

SQL> DELETE FROM TEST3 WHERE MOD(ID,3)=1;

3334 rows deleted

SQL> commit;

Commit complete

发现有碎片了

SQL> exec show\_space\_t('TEST3','auto','table','Y');

Total Blocks............................40  
Total Bytes.............................327680  
Unused Blocks...........................3  
Unused Bytes............................24576  
Last Used Ext FileId....................31  
Last Used Ext BlockId...................481921  
Last Used Block.........................5  
 \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*  
0% -- 25% free space blocks.............0  
0% -- 25% free space bytes..............0  
25% -- 50% free space blocks............31  
25% -- 50% free space bytes.............253952  
50% -- 75% free space blocks............1  
50% -- 75% free space bytes.............8192  
75% -- 100% free space blocks...........0  
75% -- 100% free space bytes............0  
Unused Blocks...........................0  
Unused Bytes............................0  
Total Blocks............................0  
Total bytes.............................0

PL/SQL procedure successfully completed

SQL>

消除碎片

SQL> alter table test3 move;

Table altered

查看碎片消除的效果

SQL> exec show\_space\_t('TEST3','auto','table','Y');

Total Blocks............................32  
Total Bytes.............................262144  
Unused Blocks...........................6  
Unused Bytes............................49152  
Last Used Ext FileId....................31  
Last Used Ext BlockId...................485065  
Last Used Block.........................2  
 \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*  
0% -- 25% free space blocks.............0  
0% -- 25% free space bytes..............0  
25% -- 50% free space blocks............0  
25% -- 50% free space bytes.............0  
50% -- 75% free space blocks............0  
50% -- 75% free space bytes.............0  
75% -- 100% free space blocks...........0  
75% -- 100% free space bytes............0  
Unused Blocks...........................0  
Unused Bytes............................0  
Total Blocks............................22  
Total bytes.............................180224

PL/SQL procedure successfully completed

SQL>

从以上看，碎片整理的效果很好！！！

**下面在测试用shrink整理碎片**

重建测试环境

SQL> drop table test3;

Table dropped

SQL>  
SQL> create table test3 as  
  2     select rownum id,  
  3     dbms\_random.string('a', round(dbms\_random.value(0,10))) col1,  
  4     trunc(sysdate) - dbms\_random.value(1, 365\*2) col2  
  5     from dual connect by rownum<=10000;

Table created

查看test3的blocks的使用

SQL> exec show\_space\_t('TEST3','auto','table','Y');

PL/SQL procedure successfully completed

制造碎片

SQL> delete from test3 where mod(id,3)=1;

3334 rows deleted

SQL> commit;

Commit complete

查看碎片情况  
SQL> exec show\_space\_t('TEST3','auto','table','Y');

Total Blocks............................40  
Total Bytes.............................327680  
Unused Blocks...........................3  
Unused Bytes............................24576  
Last Used Ext FileId....................31  
Last Used Ext BlockId...................481921  
Last Used Block.........................5  
 \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*  
0% -- 25% free space blocks.............0  
0% -- 25% free space bytes..............0  
25% -- 50% free space blocks............31  
25% -- 50% free space bytes.............253952  
50% -- 75% free space blocks............0  
50% -- 75% free space bytes.............0  
75% -- 100% free space blocks...........0  
75% -- 100% free space bytes............0  
Unused Blocks...........................0  
Unused Bytes............................0  
Total Blocks............................1  
Total bytes.............................8192

PL/SQL procedure successfully completed

用oracle10g新功能整理碎片

SQL> alter table test3 shrink space compact cascade;

alter table test3 shrink space compact cascade

ORA-10636: ROW MOVEMENT is not enabled

SQL> alter table test3 enable row movement;

Table altered

SQL> alter table test3 shrink space compact cascade;

Table altered

再次查看碎片的情况，发现还有一些碎片，整理碎片效果不好

SQL> exec show\_space\_t('TEST3','auto','table','Y');

Total Blocks............................40  
Total Bytes.............................327680  
Unused Blocks...........................3  
Unused Bytes............................24576  
Last Used Ext FileId....................31  
Last Used Ext BlockId...................481921  
Last Used Block.........................5  
 \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*  
0% -- 25% free space blocks.............1  
0% -- 25% free space bytes..............8192  
25% -- 50% free space blocks............2  
25% -- 50% free space bytes.............16384  
50% -- 75% free space blocks............0  
50% -- 75% free space bytes.............0  
75% -- 100% free space blocks...........12  
75% -- 100% free space bytes............98304  
Unused Blocks...........................0  
Unused Bytes............................0  
Total Blocks............................17  
Total bytes.............................139264

PL/SQL procedure successfully completed

上面是没降低HWM，如果载降低HWM，看看效果

SQL> alter table test3 shrink space cascade;

Table altered

SQL> exec show\_space\_t('TEST3','auto','table','Y');

Total Blocks............................24  
Total Bytes.............................196608  
Unused Blocks...........................0  
Unused Bytes............................0  
Last Used Ext FileId....................31  
Last Used Ext BlockId...................481897  
Last Used Block.........................8  
 \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*  
0% -- 25% free space blocks.............1  
0% -- 25% free space bytes..............8192  
25% -- 50% free space blocks............2  
25% -- 50% free space bytes.............16384  
50% -- 75% free space blocks............0  
50% -- 75% free space bytes.............0  
75% -- 100% free space blocks...........0  
75% -- 100% free space bytes............0  
Unused Blocks...........................0  
Unused Bytes............................0  
Total Blocks............................17  
Total bytes.............................139264

PL/SQL procedure successfully completed

看来用shrink space整理碎片不彻底，再来看看move的方式

SQL> alter table test3 move;

Table altered

SQL> exec show\_space\_t('TEST3','auto','table','Y');

Total Blocks............................32  
Total Bytes.............................262144  
Unused Blocks...........................6  
Unused Bytes............................49152  
Last Used Ext FileId....................31  
Last Used Ext BlockId...................485081  
Last Used Block.........................2  
 \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*  
0% -- 25% free space blocks.............0  
0% -- 25% free space bytes..............0  
25% -- 50% free space blocks............0  
25% -- 50% free space bytes.............0  
50% -- 75% free space blocks............0  
50% -- 75% free space bytes.............0  
75% -- 100% free space blocks...........0  
75% -- 100% free space bytes............0  
Unused Blocks...........................0  
Unused Bytes............................0  
Total Blocks............................22  
Total bytes.............................180224

PL/SQL procedure successfully completed

效果很明显，整理的很彻底

**测试结论：**

虽然alter table move和shrink space，都是通过物理调整rowid来整理碎片的，但shrink space整理的不彻底，他好像不是重组，而是尽可能的合并，随意会残留一些block无法整理

**注意：**

1.再用alter table table\_name move时，表相关的索引会失效，所以之后还要执行 alter index index\_name rebuild online; 最后重新编译数据库所有失效的对象

2. 在用alter table table\_name shrink space cascade时，他相当于alter table table\_name move和alter index index\_name rebuild online. 所以只要编译数据库失效的对象就可以

**alter table move和shrink space除了碎片整理的效果有时不一样外，还有什么其他的不同呢**

1. Move会移动高水位，但不会释放申请的空间，是在高水位以下(below HWM)的操作。  
2. shrink space 同样会移动高水位，但也会释放申请的空间，是在高水位上下(below and above HWM)都有的操作。

下面通过实验来验证

SQL> drop table test3;

Table dropped

Table created

SQL> analyze table test3 compute statistics;

Table analyzed

SQL> col segment\_name for a10;  
SQL> select us.segment\_name,us.extents,us.blocks from user\_segments us where us.segment\_name=upper('test3');

SEGMENT\_NA    EXTENTS     BLOCKS  
\---------- ---------- ----------  
TEST3               5         40

SQL> col table\_name for a10;  
SQL> select table\_name,blocks,t.empty\_blocks  from user\_tables t where t.table\_name=upper('test3');

TABLE\_NAME     BLOCKS EMPTY\_BLOCKS  
\---------- ---------- ------------  
TEST3              37            3

SQL>

从以上查询可以看出共分了5个extents，使用了37个blocks，这37也就是test3的HWM

SQL> delete from test3 where rownum<=5000;

5000 rows deleted

SQL> commit;

Commit complete

SQL> analyze table test3 compute statistics;

Table analyzed

SQL> col segment\_name for a10;  
SQL> select us.segment\_name,us.extents,us.blocks from user\_segments us where us.segment\_name=upper('test3');

SEGMENT\_NA    EXTENTS     BLOCKS  
\---------- ---------- ----------  
TEST3               5         40

SQL> col table\_name for a10;  
SQL>  select table\_name,blocks,t.empty\_blocks  from user\_tables t where t.table\_name=upper('test3');

TABLE\_NAME     BLOCKS EMPTY\_BLOCKS  
\---------- ---------- ------------  
TEST3              37            3

SQL> select count(distinct dbms\_rowid.rowid\_block\_number(rowid)) used\_blocks from test3;

USED\_BLOCKS  
\-----------  
         17

我们从查询中可以发现test3的HWM没有变换还是37blocks，tests总共空间为40blocks。经过删除后test3实际用的块是17个

下面我们用move降低下HWM

SQL> alter table test3 move;

Table altered

SQL> col segment\_name for a10;  
SQL> select us.segment\_name,us.extents,us.blocks from user\_segments us where us.segment\_name=upper('test3');

SEGMENT\_NA    EXTENTS     BLOCKS  
\---------- ---------- ----------  
TEST3               3         24

SQL> col table\_name for a10;  
SQL> select table\_name,blocks,t.empty\_blocks  from user\_tables t where t.table\_name=upper('test3');

TABLE\_NAME     BLOCKS EMPTY\_BLOCKS  
\---------- ---------- ------------  
TEST3              37            3

**user\_tables里的数据没有变化，哦，原来是忘记analyze了，从这里也可以看出user\_segments是oracle自动维护的。**

SQL> analyze table test3 compute statistics;

Table analyzed

SQL>  select us.segment\_name,us.extents,us.blocks from user\_segments us where us.segment\_name=upper('test3');

SEGMENT\_NA    EXTENTS     BLOCKS  
\---------- ---------- ----------  
TEST3               3         24

SQL> select table\_name,blocks,t.empty\_blocks  from user\_tables t where t.table\_name=upper('test3');

TABLE\_NAME     BLOCKS EMPTY\_BLOCKS  
\---------- ---------- ------------  
TEST3              20            4

SQL>

现在再来看hwm变为20了，已经降下来了啊，空间也收缩了，从40blocks降到24blocks（注意收缩到initial指定值）。  
但shrink space就收缩到存储数据的最小值，下面测试说明

创建测试表：

SQL>   create table test5 (id number) storage (initial 1m next 1m);

Table created

初始化数据

SQL>  
SQL>   begin  
  2     for i in 1..100000 loop  
  3     insert into test5 values(i);  
  4    end loop;  
  5    end;  
  6  /

PL/SQL procedure successfully completed

SQL> analyze table test5 compute statistics;

Table analyzed

SQL> select SEGMENT\_NAME,EXTENTS,BLOCKS,INITIAL\_EXTENT/1024/1024 init from user\_segments where SEGMENT\_NAME='TEST5';

SEGMENT\_NA    EXTENTS     BLOCKS       INIT  
\---------- ---------- ---------- ----------  
TEST5              17        256          1

SQL> select TABLE\_NAME,BLOCKS,EMPTY\_BLOCKS from user\_tables where table\_name='TEST5';

TABLE\_NAME     BLOCKS EMPTY\_BLOCKS  
\---------- ---------- ------------  
TEST5             180           76

可以从查询数据看出，test5初始化1m即128个blocks，但数据比较多，所以又按next参数要求扩展了1m空间，扩展了17个extents。  
这里的test5总空间大小为256个blocks，使用空间为180blocks，HWM也是180blocks

SQL> delete from test5 where rownum<=50000;

50000 rows deleted

SQL> analyze table test5 compute statistics;

Table analyzed

SQL> select SEGMENT\_NAME,EXTENTS,BLOCKS,INITIAL\_EXTENT/1024/1024 init from user\_segments where SEGMENT\_NAME='TEST5';

SEGMENT\_NA    EXTENTS     BLOCKS       INIT  
\---------- ---------- ---------- ----------  
TEST5              17        256          1

SQL> select TABLE\_NAME,BLOCKS,EMPTY\_BLOCKS from user\_tables where table\_name='TEST5';

TABLE\_NAME     BLOCKS EMPTY\_BLOCKS  
\---------- ---------- ------------  
TEST5             180           76

整理碎片，降低HWM

SQL>  alter table test5 move;

Table altered

SQL> analyze table test5 compute statistics;

Table analyzed

SQL> select SEGMENT\_NAME,EXTENTS,BLOCKS,INITIAL\_EXTENT/1024/1024 init from user\_segments where SEGMENT\_NAME='TEST5';

SEGMENT\_NA    EXTENTS     BLOCKS       INIT  
\---------- ---------- ---------- ----------  
TEST5              16        128          1

SQL> select TABLE\_NAME,BLOCKS,EMPTY\_BLOCKS from user\_tables where table\_name='TEST5';

TABLE\_NAME     BLOCKS EMPTY\_BLOCKS  
\---------- ---------- ------------  
TEST5              85           43

从上面的查询数据可以看出HWM已经从180降低到85，test5总大小从256blocks收缩到128个blocks（initial指定大小）。

下面看看用shrink space收缩空间的情况

SQL> alter table test5 enable row movement;

Table altered

SQL> alter table test5 shrink space;

Table altered

SQL> select SEGMENT\_NAME,EXTENTS,BLOCKS,INITIAL\_EXTENT/1024/1024 init from user\_segments where SEGMENT\_NAME='TEST5';

SEGMENT\_NA    EXTENTS     BLOCKS       INIT  
\---------- ---------- ---------- ----------  
TEST5              11         88          1

SQL> select TABLE\_NAME,BLOCKS,EMPTY\_BLOCKS from user\_tables where table\_name='TEST5';

TABLE\_NAME     BLOCKS EMPTY\_BLOCKS  
\---------- ---------- ------------  
TEST5              85           43

SQL>

从上面的数据可以看到test5进一步从128个blocks降低到88个blocks

**结论：**

shrink space收缩到数据存储的最小值，alter table move（不带参数）收缩到initial指定值，也可以用alter table test5 move storage(initial 500k)指定收缩的大小，这样可以达到shrink space效果

**经过以上测试，得出的两个结论，到底用哪一个命令来整理碎片，消除行迁移呢？这就要根据实际业务需要，如果你只是收缩空间，数据增长很慢，那用shrink可以但是如果数据增长很快的话，用move就比较合适，避免再重新分配空间啊**

备注：  
在10g之后，整理碎片消除行迁移的新增功能shrink space

alter table <table\_name> shrink space \[ <null> | compact | cascade \];

compact:这个参数当系统的负载比较大时可以用，不降低HWM。如果系统负载较低时，直接用alter table table\_name shrink space就一步到位了

cascade：这个参数是在shrink table的时候自动级联索引，相当于rebulid index。

**普通表：**

shrink必须开启行迁移功能。  
alter table table\_name enable row movement ;

保持HWM，相当于把块中数据打结实了  
alter table table\_name shrink space compact;

回缩表与降低HWM  
alter table table\_name shrink space;

回缩表与相关索引，降低HWM  
alter table table\_name shrink space cascade;

回缩索引与降低HWM  
alter index index\_name shrink space

虽然在10g中可以用shrink ，但也有些限制：

1). 对cluster，cluster table，或具有Long，lob类型列的对象 不起作用。  
2). 不支持具有function-based indexes 或 bitmap join indexes的表  
3). 不支持mapping 表或index-organized表。  
4). 不支持compressed 表

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