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');
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
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
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
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
------end------
alter table move跟shrink space的区别
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