--====================
-- 收缩表段(shrink space)
--====================

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] view plain copy print ?

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.

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] view plain copy print ?

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.

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] view plain copy print ?

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
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：

[sql] view plain copy print ?

ALTER TABLE ENABLE ROW MOVEMENT -->前提条件
ALTER TABLE SHRINK SPACE [ <NULL> | COMPACT | CASCADE ];
ALTER TABLE SHRINK SPACE COMPCAT; -->缩小表和索引，不移动高水位线，不释放空间
ALTER TABLE SHRINK SPACE; -->收缩表，降低高水位线;
ALTER TABLE 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 MODIFY LOB (lob_column) (SHRINK SPACE); -->收缩LOB段
ALTER INDEX IDXNAME SHRINK SPACE; -->索引段的收缩，同表段

ALTER TABLE  ENABLE ROW MOVEMENT   -->前提条件                                       
                                                                                                 
ALTER TABLE  SHRINK SPACE [  | COMPACT | CASCADE ];                            
                                                                                                 
ALTER TABLE  SHRINK SPACE COMPCAT;  -->缩小表和索引，不移动高水位线，不释放空间      
                                                                                                 
ALTER TABLE  SHRINK SPACE;     -->收缩表，降低高水位线;                              
                                                                                                 
ALTER TABLE  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  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)

[sql] view plain copy print ?

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;
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。

[sql] view plain copy print ?

--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';
--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)

[sql] view plain copy print ?

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;

ITPub博客

收缩表段(shrink space)

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