oracle数据文件的收缩

一般来讲，oracle的数据文件只扩展，不会因为tablespace上的表数据删除而自动收缩数据文件，释放磁盘空间。昨天在asktom看了一篇文章，很有意思，贴过来看看。

The only thing that'll make a file "shrink" is to alter the database and shrink the datafile - datafiles will NOT shrink on their own due to "offline/online" or anything like that.But in your case - if OEM cannot shrink it further, neither will my script.  A file contains extents of segments.  You have an extent for some allocated object out there.Say you have tables T1 and T2 in that tablespace.  T1 is represented by XT2 is represented by Yfree space in the tablespace is represented by f

You created T1 and T2, your datafile in that tablespace might look like this:XYffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffyou could shrink that file now and get rid of all of the f's.  But now table T1 grows and we have:XYXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXfffNow, you can shrink that file and get rid of just three f's (rest of the file is full of data).  Now, table T2 runs out of room and extends:XYXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXYffNow, if you shrank the file, you would lose two f's of space.  However, you drop table T1 - the datafile looks like this:fYfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffYffYou can STILL only shrink two f's away at the end - there is a Y extent way out there and we cannot shrink over it.  What you can do is:alter table t2 MOVE and hopefully the datafile will end up looking like this:YfYffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffand you can shrink the file (or just move t2 into another tablespace all together)see

http://www.oracle.com/technology/oramag/oracle/04-sep/o54asktom.html

for "What's at the end of a file"Here is an example showing what I tried to draw above:ops$tkyte@ORA9IR2> create tablespace shrink_me  2  datafile '/tmp/shrink_me.dbf' size 704k  3  segment space management manual  4  uniform size 64k  5  /Tablespace created.we have exactly 10 64k extents we can use.  (the 11th 64k block of space is used by Oracle to manage these locally managed tablespaces in the datafile)ops$tkyte@ORA9IR2>ops$tkyte@ORA9IR2> create table t1 ( x int, a char(2000) default 'a', b char(2000) default 'b', c char(2000) default 'c') tablespace shrink_me;Table created.ops$tkyte@ORA9IR2> create table t2 ( x int, a char(2000) default 'a', b char(2000) default 'b', c char(2000) default 'c') tablespace shrink_me;Table created.Each row in these tables will consume a block (8 rows/extent - but don't forget the first block is borrowed by Oracle to manage space in the segment...)ops$tkyte@ORA9IR2>ops$tkyte@ORA9IR2> select file_id, block_id first_block, block_id+blocks-1 last_block, segment_name  2    from dba_extents  3   where tablespace_name = 'SHRINK_ME'  4   union all  5  select file_id, block_id, block_id+blocks-1, 'free'  6    from dba_free_space  7   where tablespace_name = 'SHRINK_ME'  8   order by file_id, first_block  9  /   FILE_ID FIRST_BLOCK LAST_BLOCK SEGMENT_NAME---------- ----------- ---------- ------------------------------        13           9         16 T1        13          17         24 T2        13          25         88 freeso, we have the starting scenario - T1 has an extent, T2 has one and the rest of the file is "free space", now lets fill up t1:ops$tkyte@ORA9IR2> insert into t1 (x) select rownum from all_objects where rownum <= 56;56 rows created.ops$tkyte@ORA9IR2> commit;Commit complete.ops$tkyte@ORA9IR2> select file_id, block_id first_block, block_id+blocks-1 last_block, segment_name  2    from dba_extents  3   where tablespace_name = 'SHRINK_ME'  4   union all  5  select file_id, block_id, block_id+blocks-1, 'free'  6    from dba_free_space  7   where tablespace_name = 'SHRINK_ME'  8   order by file_id, first_block  9  /   FILE_ID FIRST_BLOCK LAST_BLOCK SEGMENT_NAME---------- ----------- ---------- ------------------------------        13           9         16 T1        13          17         24 T2        13          25         32 T1        13          33         40 T1        13          41         48 T1        13          49         56 T1        13          57         64 T1        13          65         72 T1        13          73         80 T1        13          81         88 free10 rows selected.we have the middle scenario - if we dropped T1 now, all of the T1's would become free space and we could shrink the file, however:ops$tkyte@ORA9IR2>ops$tkyte@ORA9IR2>ops$tkyte@ORA9IR2> insert into t2 (x) select rownum from all_objects where rownum <= 8;8 rows created.ops$tkyte@ORA9IR2> commit;Commit complete.ops$tkyte@ORA9IR2> select file_id, block_id first_block, block_id+blocks-1 last_block, segment_name  2    from dba_extents  3   where tablespace_name = 'SHRINK_ME'  4   union all  5  select file_id, block_id, block_id+blocks-1, 'free'  6    from dba_free_space  7   where tablespace_name = 'SHRINK_ME'  8   order by file_id, first_block  9  /   FILE_ID FIRST_BLOCK LAST_BLOCK SEGMENT_NAME---------- ----------- ---------- ------------------------------        13           9         16 T1        13          17         24 T2        13          25         32 T1        13          33         40 T1        13          41         48 T1        13          49         56 T1        13          57         64 T1        13          65         72 T1        13          73         80 T1        13          81         88 T210 rows selected.Now the entire tablespace is full - no more free space - but we drop t1 and get LOTS of free space:ops$tkyte@ORA9IR2> drop table t1;Table dropped.ops$tkyte@ORA9IR2> select file_id, block_id first_block, block_id+blocks-1 last_block, segment_name  2    from dba_extents  3   where tablespace_name = 'SHRINK_ME'  4   union all  5  select file_id, block_id, block_id+blocks-1, 'free'  6    from dba_free_space  7   where tablespace_name = 'SHRINK_ME'  8   order by file_id, first_block  9  /   FILE_ID FIRST_BLOCK LAST_BLOCK SEGMENT_NAME---------- ----------- ---------- ------------------------------        13           9         16 free        13          17         24 T2        13          25         80 free        13          81         88 T2the tablespace is 80% empty - but we cannot shrink it - we cannot shrink it at all, let alone to 300k - because the LAST EXTENT is taken by T2ops$tkyte@ORA9IR2> alter database datafile '/tmp/shrink_me.dbf' resize 300k;alter database datafile '/tmp/shrink_me.dbf' resize 300k*ERROR at line 1:ORA-03297: file contains used data beyond requested RESIZE valuebut lets move T2 around...ops$tkyte@ORA9IR2> alter table t2 move;Table altered.ops$tkyte@ORA9IR2>ops$tkyte@ORA9IR2> select file_id, block_id first_block, block_id+blocks-1 last_block, segment_name  2    from dba_extents  3   where tablespace_name = 'SHRINK_ME'  4   union all  5  select file_id, block_id, block_id+blocks-1, 'free'  6    from dba_free_space  7   where tablespace_name = 'SHRINK_ME'  8   order by file_id, first_block  9  /   FILE_ID FIRST_BLOCK LAST_BLOCK SEGMENT_NAME---------- ----------- ---------- ------------------------------        13           9         16 T2        13          17         24 free        13          25         32 T2        13          33         88 freenow we have lots of free space at the end of the file and we can resize:ops$tkyte@ORA9IR2> alter database datafile '/tmp/shrink_me.dbf' resize 300k;Database altered.ops$tkyte@ORA9IR2> select file_id, block_id first_block, block_id+blocks-1 last_block, segment_name  2    from dba_extents  3   where tablespace_name = 'SHRINK_ME'  4   union all  5  select file_id, block_id, block_id+blocks-1, 'free'  6    from dba_free_space  7   where tablespace_name = 'SHRINK_ME'  8   order by file_id, first_block  9  /   FILE_ID FIRST_BLOCK LAST_BLOCK SEGMENT_NAME---------- ----------- ---------- ------------------------------        13           9         16 T2        13          17         24 free        13          25         32 T2don't forget, if you move a table, you have to then rebuild the indexes