在Oracle中，有些表由于频繁的插入和删除数据，导致高水位过高，表的碎片也很高，如何判断呢？
drop table test purge;
create table test as select * from dba_objects;insert into test select * from test;
insert into test select * from test;
commit;
select trunc(real_size('TEST')/1024/1024,2) real_size,
bytes /1024/1024||'M' seg_size,
trunc((1 - real_size('TEST')/ bytes) * 100,2) || '%' frag_ratio
from user_segments s where s.segment_name='TEST';
REAL_SIZE SEG_SIZE FRAG_RATIO
---------- ----------- -------------
32.8 33M .58%
select object_type,count(1)
from test
group by object_type
having count(1) >5000
order by count(1) desc;
OBJECT_TYPE COUNT(1)
------------------- ----------
SYNONYM 111492
JAVA CLASS 91668
VIEW 20656
INDEX 15604
TABLE 11580
TYPE 11292
PACKAGE 5632
PACKAGE BODY 5344
--可以看到随着数据的不断删除，碎片率在升高
delete from test where object_type in('SYNONYM','JAVA CLASS');
commit;
select trunc(real_size('TEST')/1024/1024,2) || '%' frag_ratio
from user_segments s where s.segment_name='TEST';
REAL_SIZE SEG_SIZE FRAG_RATIO
---------- ----------- -------------
12 33M 63.62%
delete from test where object_type in('VIEW','INDEX','TABLE','TYPE');
commit;
select trunc(real_size('TEST')/1024/1024,2) || '%' frag_ratio
from user_segments s where s.segment_name='TEST';
REAL_SIZE SEG_SIZE FRAG_RATIO
---------- ----------- -------------
6.59 33M 80.02%
--把表收缩一下，碎片率下降了
alter table test enable row movement;
alter table test shrink space;
select trunc(real_size('TEST')/1024/1024,2) || '%' frag_ratio
from user_segments s where s.segment_name='TEST';
REAL_SIZE SEG_SIZE FRAG_RATIO
---------- ----------- -------------
3.2 3.25M 1.53%
附录是Tom kyte提供的检查脚本：
CREATE OR REPLACE FUNCTION REAL_SIZE( P_SEGNAME IN VARCHAR2,P_OWNER IN VARCHAR2 DEFAULT USER,P_TYPE IN VARCHAR2 DEFAULT 'TABLE') RETURN NUMBER AUTHID CURRENT_USER AS 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_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; T_TOTAL_BYTES NUMBER; T_FS_BYTES NUMBER; P_PART_NAME VARCHAR2(30); BEGIN 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_PART_NAME); DBMS_SPACE.UNUSED_SPACE( P_OWNER,L_TOTAL_BLOCKS,L_TOTAL_BYTES,L_UNUSED_BLOCKS,L_UNUSED_BYTES,L_LASTUSEDEXTFILEID,L_LASTUSEDEXTBLOCKID,L_LAST_USED_BLOCK,P_PART_NAME); T_FS_BYTES := L_FS1_BYTES * 0.25 / 2 + L_FS2_BYTES * (0.5 + 0.25) / 2 +L_FS3_BYTES * (0.75 + 0.5) / 2 +L_FS4_BYTES * (1 + 0.75) / 2 + L_UNUSED_BYTES; T_TOTAL_BYTES := L_TOTAL_BYTES; RETURN T_TOTAL_BYTES-T_FS_BYTES; EXCEPTION WHEN OTHERS THEN RETURN 1; END;
