--====================
-- 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> 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;
七、快捷参考
有关性能优化请参考
有关ORACLE体系结构请参考
Oracle联机重做日志文件(ONLINE LOG FILE)
Oracle实例和Oracle数据库(Oracle体系结构)
有关闪回特性请参考
Oracle闪回特性(FLASHBACK DATABASE)
Oracle闪回特性(FLASHBACK DROP & RECYCLEBIN)
Oracle闪回特性(Flashback Query、FlashbackTable)
Oracle闪回特性(Flashback Version、Flashback Transaction)
有关基于用户管理的备份和备份恢复的概念请参考
Oracle基于用户管理恢复的处理(详细描述了介质恢复及其处理)
有关RMAN的备份恢复与管理请参考
RMAN 备份路径困惑(使用plus archivelog时)
有关ORACLE故障请参考
对参数FAST_START_MTTR_TARGET= 0 的误解及设定
有关ASM请参考
有关SQL/PLSQL请参考
SQL 基础--> 集合运算(UNION与UNION ALL)
SQL 基础--> 层次化查询(STARTBY ... CONNECT BY PRIOR)
SQL 基础--> ROLLUP与CUBE运算符实现数据汇总
有关ORACLE其它特性
使用OEM,SQL*Plus,iSQL*Plus 管理Oracle实例
日志记录模式(LOGGING、FORCE LOGGING 、NOLOGGING)
使用外部表管理Oracle 告警日志(ALAERT_$SID.LOG)
簇表及簇表管理(Index clustered tables)
ORACLE_SID、DB_NAME、INSTANCE_NAME、DB_DOMIAN、GLOBAL_NAME
Oracle补丁全集 (Oracle 9i 10g 11g Path)
时间： 2024-07-28 17:29:37