收缩表段(shrink space) -

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收缩表段(shrink space)

--====================
-- 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

Oracle硬解析与软解析

共享池的调整与优化(Sharedpool Tuning)

Buffercache 的调整与优化(一)

Oracle表缓存(cachingtable)的使用

有关ORACLE体系结构请参考

Oracle表空间与数据文件

Oracle密码文件

Oracle参数文件

Oracle联机重做日志文件(ONLINE LOG FILE)

Oracle控制文件(CONTROLFILE)