收缩表段(shrink space)

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-- 收缩表段(shrink space)

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一、表的增长方式

当表被创建后，随着记录的不断插入，组成表的区间会被填满，如果启用了自动扩展，则当区间填满后，会分配新的区间。假定高水

位线随着记录的增加从最左端往右端来移动，当到底部区间的尾端时，则新的区间将会被分配。

二、表可收缩的原理

随着记录的增加高水位线不断的右移，记录的删除不会导致高水位线往回(左)移动

删除记录后的空闲空间(高水位线左侧)尽管可以使用，但其稀疏性导致空间空闲

注：完整的表扫描所耗费的时间不会因为记录的减少(删除)而减少

三、使用 alter table tbname shrink space 来收缩表段

1. 实现原理

实质上构造一个新表(在内部表现为一系列的DML操作,即将副本插入新位置，删除原来位置的记录)

靠近末尾处(右端)数据块中的记录往开始处(左端)的空闲空间处移动(DML操作)，不会引起DML触发器

当所有可能的移动被完成，高水位线将会往左端移动(DDL操作)

新的高水位线右边的空闲空间被释放(DDL操作)

2. 实现前提条件

必须启用行记录转移(enable row movement)

仅仅适用于堆表,且位于自动段空间管理的表空间(堆表包括:标准表,分区表,物化视图容器,物化视图日志表)

3. 不能实现收缩的表

群集表

具有LONG类型列的表

LOB段(尽管表本身可以被缩小)，注，10gR2以后版本支持对LOB段的收缩

具有基于提交的物化视图的表(因为禁用了触发器)

具有rowid物化视图的表(因为rowid发生了变化)

IOT映射表IOT溢出段

索引基于函数的表

未启用行记录转移的堆表

4. 段收缩的优点

提高缓存利用率，提高OLTP的性能

减少磁盘I/O，提高访问速度，节省磁盘空间

段收缩是在线的，索引在段收缩期间维护，不要求额外的磁盘空间

5. 两个选项

cascade:缩小表及其索引，并移动高水位线，释放空间

compact:仅仅是缩小表和索引，并不移动高水位线，不释放空间

alter table tbname shrink space 相当于带cascade参数

四、实战演习

1. 查看需要收缩的表段的基本情况，此处为表big_table

SQL> col SEGMENT_NAME format a20

SQL> select OWNER,SEGMENT_NAME,BYTES/1024/1024,BLOCKS from dba_segments

2 where owner='SCOTT' and SEGMENT_NAME='BIG_TABLE';

OWNER SEGMENT_NAME BYTES/1024/1024 BLOCKS

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

SCOTT BIG_TABLE 120 15360

SQL> select table_name,blocks,empty_blocks from dba_tables

2 where table_name='big_table' and owner='scott'; -->未更新统计信息前BLOCKS，EMPTY_BLOCKS列为空

TABLE_NAME BLOCKS EMPTY_BLOCKS

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

BIG_TABLE

SQL> exec dbms_stats.gather_table_stats(ownname=>'SCOTT',tabname=>'BIG_TABLE',estimate_percent=>30);

PL/SQL procedure successfully completed.

SQL> select TABLE_NAME,BLOCKS,EMPTY_BLOCKS from dba_tables

2 where TABLE_NAME='BIG_TABLE' and owner='SCOTT';

TABLE_NAME BLOCKS EMPTY_BLOCKS

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

BIG_TABLE 14590 0 -->使用gather_table_stats时不会统计EMPTY_BLOCKS块

SQL> analyze table big_table compute statistics; -->使用analyze更新统计信息后EMPTY_BLOCKS得到数据

Table analyzed.

SQL> select TABLE_NAME,BLOCKS,EMPTY_BLOCKS from dba_tables

2 where TABLE_NAME='BIG_TABLE' and owner='SCOTT';

TABLE_NAME BLOCKS EMPTY_BLOCKS

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

BIG_TABLE 14590 770

SQL> set serveroutput on;

SQL> exec show_space('BIG_TABLE','SCOTT'); -->使用show_space过程或的BIG_TABLE上的空间分配信息

Unformatted Blocks ..................... 0

FS1 Blocks (0-25) ...................... 0 -->空闲度为0-25%的块数。FS1,FS2,FS3,FS4为空闲度所占的百分比

FS2 Blocks (25-50) ..................... 0

FS3 Blocks (50-75) ..................... 0

FS4 Blocks (75-100)..................... 0

Full Blocks ............................ 14,427

Total Blocks............................ 15,360

Total Bytes............................. 125,829,120

Total MBytes............................ 120

Unused Blocks........................... 770

Unused Bytes............................ 6,307,840

Last Used Ext FileId.................... 4

Last Used Ext BlockId................... 16,521

Last Used Block......................... 254

PL/SQL procedure successfully completed.

2. 删除记录之后，进行收缩表段

SQL> delete from big_table where owner in ('SCOTT','SYSTEM'); -->删除记录

8715 rows deleted.

SQL> commit;

Commit complete.

SQL> alter table big_table shrink space; -->实施shrink,提示没有启用ROW MOVEMENT

alter table big_table shrink space

ERROR at line 1:

ORA-10636: ROW MOVEMENT is not enabled

SQL> alter table big_table enable row movement; -->开启row movement

Table altered.

SQL> alter table big_table shrink space; -->shrink成功

Table altered.

SQL> exec show_space('BIG_TABLE','SCOTT'); -->从下面的结果中可以看到块数以及总大小已经变小

Unformatted Blocks ..................... 0

FS1 Blocks (0-25) ...................... 1

FS2 Blocks (25-50) ..................... 1

FS3 Blocks (50-75) ..................... 0

FS4 Blocks (75-100)..................... 0

Full Blocks ............................ 14,318

Total Blocks............................ 14,488

Total Bytes............................. 118,685,696

Total MBytes............................ 113

Unused Blocks........................... 5

Unused Bytes............................ 40,960

Last Used Ext FileId.................... 4

Last Used Ext BlockId................... 16,521

Last Used Block......................... 147

PL/SQL procedure successfully completed.

3. 验证cascade与compact的差异

SQL> delete from big_table where rownum<8000; -->再次删除一些记录

7999 rows deleted.

SQL> alter table big_table shrink space compact; -->使用compact方式收缩表段

Table altered.

SQL> exec show_space('BIG_TABLE','SCOTT');

Unformatted Blocks ..................... 0

FS1 Blocks (0-25) ...................... 1

FS2 Blocks (25-50) ..................... 2

FS3 Blocks (50-75) ..................... 0

FS4 Blocks (75-100)..................... 103

Full Blocks ............................ 14,214 --仅有的变化为14318-14214=104块，即完全填满的数据块减少了104块

Total Blocks............................ 14,488 --数据的总块数及总大小并没有减少，即未移动高水位线

Total Bytes............................. 118,685,696

Total MBytes............................ 113

Unused Blocks........................... 5

Unused Bytes............................ 40,960

Last Used Ext FileId.................... 4

Last Used Ext BlockId................... 16,521

Last Used Block......................... 147

PL/SQL procedure successfully completed.

SQL> alter table big_table shrink space cascade; -->使用cascade方式收缩，

Table altered.

SQL> exec show_space('BIG_TABLE','SCOTT');

Unformatted Blocks ..................... 0

FS1 Blocks (0-25) ...................... 1

FS2 Blocks (25-50) ..................... 2

FS3 Blocks (50-75) ..................... 0

FS4 Blocks (75-100)..................... 0

Full Blocks ............................ 14,214

Total Blocks............................ 14,384 -->总块数及总大小均已减少

Total Bytes............................. 117,833,728

Total MBytes............................ 112

Unused Blocks........................... 4

Unused Bytes............................ 32,768

Last Used Ext FileId.................... 4

Last Used Ext BlockId................... 16,521

Last Used Block......................... 44

PL/SQL procedure successfully completed.

-->收缩之后索引依然有效

SQL> select OWNER,INDEX_NAME,STATUS from dba_indexes where TABLE_NAME='BIG_TABLE';

OWNER INDEX_NAME STATUS

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

SCOTT BIG_TABLE_PK VALID

五、语法总结：

ALTER TABLE <table_name> ENABLE ROW MOVEMENT -->前提条件

ALTER TABLE <table_name> SHRINK SPACE [ <NULL> | COMPACT | CASCADE ];

ALTER TABLE <table_name> SHRINK SPACE COMPCAT; -->缩小表和索引，不移动高水位线，不释放空间

ALTER TABLE <table_name> SHRINK SPACE; -->收缩表，降低高水位线;

ALTER TABLE <table_name> SHRINK SPACE CASCADE; -->收缩表，降低高水位线，并且相关索引也要收缩一下

ALTER TABLE <table_name> MODIFY LOB (lob_column) (SHRINK SPACE); -->收缩LOB段

ALTER INDEX IDXNAME SHRINK SPACE; -->索引段的收缩，同表段

六、批量收缩脚本

1. 普通表(根据相应需求修改下面的语句生产相应脚本)

select'alter table '||table_name||' enable row movement;'

||chr(10)||'alter table '||table_name||' shrink space;'||chr(10)from user_tables;

select'alter index '||index_name||' shrink space;'||chr(10)from user_indexes;

2. 分区表的处理

分区表进行shrink space时发生ORA-10631错误.shrink space有一些限制.

在表上建有函数索引（包括全文索引）会失败。

--根据相应需求修改下面的语句生产相应脚本

select 'alter table '||owner||'.'||table_name||' enable row movement;'

||chr(10)||'alter table '||owner||'.'||table_name||' shrink space;'||chr(10) from dba_tables

where owner=upper('&input_owner');

select 'alter index '||owner||'.'||index_name||' shrink space;'

||chr(10) from dba_indexes where uniqueness='NONUNIQUE' ;

select 'alter table '||owner||'.'||segment_name||' modify partition '||partition_name||' shrink space;'

||chr(10) from dba_segments where segment_type='TABLE PARTITION';

3. 附show_space脚本(来自Tom大师)

CREATE OR REPLACE PROCEDURE show_space

(

p_segname IN VARCHAR2,

p_owner IN VARCHAR2 DEFAULT USER,

p_type IN VARCHAR2 DEFAULT 'TABLE',

p_partition IN VARCHAR2 DEFAULT NULL

)

-- this procedure uses authid current user so it can query DBA_*

-- views using privileges from a ROLE, and so it can be installed

-- once per database, instead of once per user who wanted to use it

AUTHID CURRENT_USER AS

l_free_blks NUMBER;

l_total_blocks NUMBER;

l_total_bytes NUMBER;

l_unused_blocks NUMBER;

l_unused_bytes NUMBER;

l_LastUsedExtFileId NUMBER;

l_LastUsedExtBlockId NUMBER;

l_LAST_USED_BLOCK NUMBER;

l_segment_space_mgmt VARCHAR2(255);

l_unformatted_blocks NUMBER;

l_unformatted_bytes NUMBER;

l_fs1_blocks NUMBER;

l_fs1_bytes NUMBER;

l_fs2_blocks NUMBER;

l_fs2_bytes NUMBER;

l_fs3_blocks NUMBER;

l_fs3_bytes NUMBER;

l_fs4_blocks NUMBER;

l_fs4_bytes NUMBER;

l_full_blocks NUMBER;

l_full_bytes NUMBER;

-- inline procedure to print out numbers nicely formatted

-- with a simple label

PROCEDURE p

(

p_label IN VARCHAR2,

p_num IN NUMBER

) IS

BEGIN

dbms_output.put_line(rpad(p_label, 40, '.') ||

to_char(p_num, '999,999,999,999'));

END;

BEGIN

-- this query is executed dynamically in order to allow this procedure

-- to be created by a user who has access to DBA_SEGMENTS/TABLESPACES

-- via a role as is customary.

-- NOTE: at runtime, the invoker MUST have access to these two

-- views!

-- this query determines if the object is an ASSM object or not

BEGIN

EXECUTE IMMEDIATE 'select ts.segment_space_management

from dba_segments seg, dba_tablespaces ts

where seg.segment_name = :p_segname

and (:p_partition is null or

seg.partition_name = :p_partition)

and seg.owner = :p_owner

and seg.tablespace_name = ts.tablespace_name'

INTO l_segment_space_mgmt

USING p_segname, p_partition, p_partition, p_owner;

EXCEPTION

WHEN too_many_rows THEN

dbms_output.put_line('This must be a partitioned table, use p_partition => ');

RETURN;

END;

-- if the object is in an ASSM tablespace, we must use this API

-- call to get space information, otherwise we use the FREE_BLOCKS

-- API for the user-managed segments

IF l_segment_space_mgmt = 'AUTO' THEN

dbms_space.space_usage(p_owner,

p_segname,

p_type,

l_unformatted_blocks,

l_unformatted_bytes,

l_fs1_blocks,

l_fs1_bytes,

l_fs2_blocks,

l_fs2_bytes,

l_fs3_blocks,

l_fs3_bytes,

l_fs4_blocks,

l_fs4_bytes,

l_full_blocks,

l_full_bytes,

p_partition);

p('Unformatted Blocks ', l_unformatted_blocks);

p('FS1 Blocks (0-25) ', l_fs1_blocks);

p('FS2 Blocks (25-50) ', l_fs2_blocks);

p('FS3 Blocks (50-75) ', l_fs3_blocks);

p('FS4 Blocks (75-100)', l_fs4_blocks);

p('Full Blocks ', l_full_blocks);

ELSE

dbms_space.free_blocks(segment_owner => p_owner,

segment_name => p_segname,

segment_type => p_type,

freelist_group_id => 0,

free_blks => l_free_blks);

p('Free Blocks', l_free_blks);

END IF;

-- and then the unused space API call to get the rest of the

-- information

dbms_space.unused_space(segment_owner => p_owner,

segment_name => p_segname,

segment_type => p_type,

partition_name => p_partition,

total_blocks => l_total_blocks,

total_bytes => l_total_bytes,

unused_blocks => l_unused_blocks,

unused_bytes => l_unused_bytes,

LAST_USED_EXTENT_FILE_ID => l_LastUsedExtFileId,

LAST_USED_EXTENT_BLOCK_ID => l_LastUsedExtBlockId,

LAST_USED_BLOCK => l_LAST_USED_BLOCK);

p('Total Blocks', l_total_blocks);

p('Total Bytes', l_total_bytes);

p('Total MBytes', trunc(l_total_bytes / 1024 / 1024));

p('Unused Blocks', l_unused_blocks);

p('Unused Bytes', l_unused_bytes);

p('Last Used Ext FileId', l_LastUsedExtFileId);

p('Last Used Ext BlockId', l_LastUsedExtBlockId);

p('Last Used Block', l_LAST_USED_BLOCK);

END;

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