SQL中如何释放大数据量的lob字段空间，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
SQL> create tablespace ts_lob datafile '/u01/app/oracle/oradata/DBdb/ts_lob.dbf' size 500m autoextend off;
Tablespace created.
--scott用户创建测试表lob1:
SQL> grant dba to scott;
Grant succeeded.
SQL> conn scott/tiger;
Connected.
SQL> create table lob1(line number,text clob) tablespace ts_lob;
Table created.
SQL> insert into lob1 select line,text from dba_source;
637502 rows created.
SQL> insert into lob1 select * from lob1;
637502 rows created.
SQL> select count(*) from lob1;
COUNT(*)
----------
1275004
SQL> commit;
Commit complete.
--查询表大小(包含表和lob字段)
select (select nvl(sum(s.bytes/1024/1204), 0) -- the table segment size
from dba_segments s
where s.owner = upper('SCOTT')
and (s.segment_name = upper('LOB1'))) +
(select nvl(sum(s.bytes/1024/1024), 0) -- the lob segment size
from dba_segments s, dba_lobs l
where s.owner = upper('SCOTT')
and (l.segment_name = s.segment_name and
l.table_name = upper('LOB1') and
l.owner = upper('SCOTT'))) +
(select nvl(sum(s.bytes/1024/1024), 0) -- the lob index size
from dba_segments s, dba_indexes i
where s.owner = upper('SCOTT')
and (i.index_name = s.segment_name and
i.table_name = upper('LOB1') and index_type = 'LOB' and
i.owner = upper('SCOTT'))) "total_table_size_M"
FROM DUAL;
total_table_size_M
------------------
239.966154
--查询表大小(不包含lob字段)
col SEGMENT_NAME for a30
col PARTITION_NAME for a30
SQL> select OWNER,SEGMENT_NAME,PARTITION_NAME,BYTES/1024/1024 M from dba_segments where segment_name='LOB1' and owner='SCOTT';
OWNER SEGMENT_NAME PARTITION_NAME M
------------------------------ ------------------------------ ------------------------------ ----------
SCOTT LOB1 208
--查询表大小(只包含lob字段)
set lines 200 pages 999
col owner for a15
col TABLE_NAME for a20
col COLUMN_NAME for a30
col SEGMENT_NAME for a30
select a.owner,
a.table_name,
a.column_name,
b.segment_name,
b.segment_type,
ROUND(b.BYTES / 1024 / 1024)
from dba_lobs a, dba_segments b
where a.segment_name = b.segment_name
and a.owner = 'SCOTT'
and a.table_name = 'LOB1'
union all
select a.owner,
a.table_name,
a.column_name,
b.segment_name,
b.segment_type,
ROUND(b.BYTES / 1024 / 1024)
from dba_lobs a, dba_segments b
where a.index_name = b.segment_name
and a.owner = 'SCOTT'
and a.table_name = 'LOB1';
OWNER TABLE_NAME COLUMN_NAME SEGMENT_NAME SEGMENT_TYPE ROUND(B.BYTES/1024/1024)
--------------- -------------------- ------------------------------ ------------------------------ ------------------ ------------------------
SCOTT LOB1 TEXT SYS_LOB0000089969C00002$$ LOBSEGMENT 63
SCOTT LOB1 TEXT SYS_IL0000089969C00002$$ LOBINDEX 0
--查询ts_lob表空间的表大小排行
SQL> select * from (select SEGMENT_NAME,sum(bytes)/1024/1024 sx from dba_segments
where tablespace_name='TS_LOB' group by segment_name )
order by sx desc;
SEGMENT_NAME SX
------------------------------ ----------
LOB1 208
SYS_LOB0000089969C00002$$ 63
SYS_IL0000089969C00002$$ .0625
--查询lob字段SCOTT_LOB0000089963C00002$$ 、SCOTT_IL0000089963C00002$$:
SQL> col object_name for a30
SQL> select OWNER,OBJECT_NAME,OBJECT_TYPE from dba_objects where OBJECT_NAME in('SYS_LOB0000089969C00002$$','SYS_IL0000089969C00002$$');
OWNER OBJECT_NAME OBJECT_TYPE
--------------- ------------------------------ -------------------
SCOTT SYS_IL0000089969C00002$$ INDEX
SCOTT SYS_LOB0000089969C00002$$ LOB
SQL> select OWNER,TABLE_NAME,COLUMN_NAME,SEGMENT_NAME,TABLESPACE_NAME,INDEX_NAME from dba_lobs where segment_name in('SYS_LOB0000089969C00002$$','SYS_IL0000089969C00002$$');
OWNER TABLE_NAME COLUMN_NAME SEGMENT_NAME TABLESPACE_NAME INDEX_NAME
--------------- -------------------- ------------------------------ ------------------------------ ------------------------------ ------------------------------
SCOTT LOB1 TEXT SYS_LOB0000089969C00002$$ TS_LOB SYS_IL0000089969C00002$$
SQL>
SQL> select SEGMENT_NAME,bytes /1024/1024 sx from dba_segments where tablespace_name='TS_LOB' and SEGMENT_NAME in('SYS_LOB0000089969C00002$$','SYS_IL0000089969C00002$$');
SEGMENT_NAME SX
------------------------------ ----------
SYS_LOB0000089969C00002$$ 63
SYS_IL0000089969C00002$$ .0625
一、先试着删除lob字段：
SQL> alter table scott.lob1 drop (text);
Table altered.
SQL> select SEGMENT_NAME,bytes /1024/1024 sx from dba_segments where tablespace_name='TS_LOB' and SEGMENT_NAME in('SYS_LOB0000089969C00002$$','SYS_IL0000089969C00002$$');
no rows selected
SQL> select * from (select SEGMENT_NAME,sum(bytes)/1024/1024 sx from dba_segments where tablespace_name='TS_LOB' group by segment_name ) order by sx desc;
SEGMENT_NAME SX
------------------------------ ----------
LOB1 208
发现删除lob字段可以释放表空间。
--再次添加LOB字段：
SQL> alter table scott.lob1 add (text clob);
Table altered.
SQL> select * from (select SEGMENT_NAME,sum(bytes)/1024/1024 sx from dba_segments where tablespace_name='TS_LOB' group by segment_name ) order by sx desc;
SEGMENT_NAME SX
------------------------------ ----------
LOB1 208
SYS_LOB0000089969C00002$$ .0625
SYS_IL0000089969C00002$$ .0625
二、再次插入数据：
SQL> insert into scott.lob1 select LINE,text from dba_source;
637502 rows created.
SQL> insert into scott.lob1 select LINE,text from dba_source;
637502 rows created.
SQL> commit;
Commit complete.
SQL> select * from (select SEGMENT_NAME,sum(bytes)/1024/1024 sx from dba_segments where tablespace_name='TS_LOB' group by segment_name ) order by sx desc;
SEGMENT_NAME SX
------------------------------ ----------
LOB1 208
SYS_LOB0000089969C00002$$ 63
SYS_IL0000089969C00002$$ .0625
--接着试着truncate表LOB1
SQL> truncate table scott.lob1;
Table truncated.
SQL> select * from (select SEGMENT_NAME,sum(bytes)/1024/1024 sx from dba_segments where tablespace_name='TS_LOB' group by segment_name ) order by sx desc;
SEGMENT_NAME SX
------------------------------ ----------
LOB1 .0625
SYS_LOB0000089969C00002$$ .0625
SYS_IL0000089969C00002$$ .0625
truncate表也可以释放lob字段数据；
三、再次插入数据：
SQL> insert into scott.lob1 select LINE,text from dba_source;
637502 rows created.
SQL> insert into scott.lob1 select LINE,text from dba_source;
637502 rows created.
SQL> commit;
Commit complete.
SQL>
SQL> select * from (select SEGMENT_NAME,sum(bytes)/1024/1024 sx from dba_segments where tablespace_name='TS_LOB' group by segment_name ) order by sx desc;
SEGMENT_NAME SX
------------------------------ ----------
LOB1 184
SYS_LOB0000089969C00002$$ 63
SYS_IL0000089969C00002$$ .0625
使用delete方式删除数据，实际上物理块还是被占用，高水位没有下降。
SQL> delete scott.lob1;
1275004 rows deleted.
SQL>
SQL> select * from (select SEGMENT_NAME,sum(bytes)/1024/1024 sx from dba_segments where tablespace_name='TS_LOB' group by segment_name ) order by sx desc;
SEGMENT_NAME SX
------------------------------ ----------
LOB1 184
SYS_LOB0000089969C00002$$ 63
SYS_IL0000089969C00002$$ .75
SQL> select count(*) from scott.lob1;
COUNT(*)
----------
0
SQL> truncate table scott.lob1;
Table truncated.
SQL> select * from (select SEGMENT_NAME,sum(bytes)/1024/1024 sx from dba_segments where tablespace_name='TS_LOB' group by segment_name ) order by sx desc;
SEGMENT_NAME SX
------------------------------ ----------
LOB1 .0625
SYS_LOB0000089969C00002$$ .0625
SYS_IL0000089969C00002$$ .0625
结论：在删除lob字段的大数据量时，可以采用重建表（CTAS)、删除lob字段再重建alter table table_name drop (column)、导出导入（只导出元数据）、或者直接truncate全表删除全表（包括lob)降低高水位。
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