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;