剖析shrink 操作原理
在上一篇介绍了move 的操作，这篇介绍下shrink 操作:
在oracle 10g之后，推出了，shrink 操作，使用shrink 操作的时候索引可以不失效，这个对应用系统的影响要小不少。
SQL> create table daodao (id int, name char(2000)) tablespace users;
Table created.
SQL> insert into daodao select rownum,'ddd' from dba_objects where rownum <=50;
50 rows created.
SQL> commit;
Commit complete.
SQL> show user;
USER is "SYS"
SQL> select header_file,header_block from dba_segments where segment_name ='DAODAO';
HEADER_FILE HEADER_BLOCK
----------- ------------
4 2387
SQL> set pages 2000;
SQL> select Dbms_Rowid.rowid_block_number(rowid),count(1) from daodao group by Dbms_Rowid.rowid_block_number(rowid) ;
DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) COUNT(1)
------------------------------------ ----------
2388 3
2389 3
2398 3
2390 3
2393 3
2395 3
2399 3
2400 3
2404 3
2405 3
2391 3
2397 3
2392 3
2406 3
2407 2
2394 3
2396 3
17 rows selected.
SQL> select header_file,header_block from dba_segments where segment_name ='DAODAO';
HEADER_FILE HEADER_BLOCK
----------- ------------
4 2387
SQL> exec show_space('DAODAO');
Unformatted Blocks ..................... 0
FS1 Blocks (0-25) ...................... 0
FS2 Blocks (25-50) ..................... 1 --- 该块为2407 存储2行的块，现在还剩余25%-50%的剩余空间
FS3 Blocks (50-75) ..................... 0
FS4 Blocks (75-100)..................... 3 --- 3个为已格式化的空块
Full Blocks ............................ 16 --- 16个为已经满的块
Total Blocks............................ 24 --- 消耗的总块数为24个
Total Bytes............................. 196,608
Total MBytes............................ 0
Unused Blocks........................... 0
Unused Bytes............................ 0
Last Used Ext BlockId................... 2,401 --- 最后使用的区头的blockid 对应dba_extents 的block_id
Last Used Block......................... 8
PL/SQL procedure successfully completed.
SQL> alter table daodao enable row movement;
Table altered.
SQL> delete from daodao where mod(id,2)=1;
25 rows deleted.
SQL> commit;
Commit complete.
SQL> select Dbms_Rowid.rowid_block_number(rowid),count(1) from daodao group by Dbms_Rowid.rowid_block_number(rowid) ;
DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) COUNT(1)
------------------------------------ ----------
2388 1
2389 1
2398 2
2390 2
2393 2
2395 1
2399 1
2400 2
2404 2
2405 1
2391 1
2397 1
2392 2
2406 2
2402 1
2394 1
2396 2
17 rows selected.
SQL> exec show_space('DAODAO');
Unformatted Blocks ..................... 0
FS1 Blocks (0-25) ...................... 0
FS2 Blocks (25-50) ..................... 8
FS3 Blocks (50-75) ..................... 9
FS4 Blocks (75-100)..................... 3
Full Blocks ............................ 0
Total Blocks............................ 24
Total Bytes............................. 196,608
Total MBytes............................ 0
Unused Blocks........................... 0
Unused Bytes............................ 0
Last Used Ext BlockId................... 2,401
Last Used Block......................... 8
PL/SQL procedure successfully completed.
SQL> alter table daodao shrink space compact;
Table altered.
SQL> select Dbms_Rowid.rowid_block_number(rowid),count(1) from daodao group by Dbms_Rowid.rowid_block_number(rowid) ;
DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) COUNT(1)
------------------------------------ ----------
2388 4
2389 4
2390 4
2393 4
2391 4
2392 4
2394 1
7 rows selected.
SQL> select header_file,header_block from dba_segments where segment_name ='DAODAO';
HEADER_FILE HEADER_BLOCK
----------- ------------
4 2387
-- 从这里可以看到和move 操作完全不一样，这里段头没有改变，move 为什么需要多一倍空间，就是因为move 执行的是创建一个表，迁移，然后再
删除原表的操作，而shrink 则不一样，他完全是在自己的表里面进行操作，所以他不需要多一倍空间。
SQL> exec show_space('DAODAO');
Unformatted Blocks ..................... 0
FS1 Blocks (0-25) ...................... 1
FS2 Blocks (25-50) ..................... 0
FS3 Blocks (50-75) ..................... 1
FS4 Blocks (75-100)..................... 13
Full Blocks ............................ 5
Total Blocks............................ 24 ---高水位没有降低
Total Bytes............................. 196,608
Total MBytes............................ 0
Unused Blocks........................... 0
Unused Bytes............................ 0
Last Used Ext BlockId................... 2,401 ---最后一个区块没有移动
Last Used Block......................... 8
PL/SQL procedure successfully completed.
--- 这里就是为什么说shrink space compact 是轻量级的shrink ，只是使得数据块之间更加夯实了，实际上高水位没有降低。
SQL> alter table daodao shrink space ;
Table altered.
SQL> exec show_space('DAODAO');
Unformatted Blocks ..................... 0
FS1 Blocks (0-25) ...................... 0
FS2 Blocks (25-50) ..................... 0
FS3 Blocks (50-75) ..................... 0
FS4 Blocks (75-100)..................... 1
Full Blocks ............................ 6
Total Blocks............................ 16
Total Bytes............................. 131,072
Total MBytes............................ 0
Unused Blocks........................... 6
Unused Bytes............................ 49,152
Last Used Ext BlockId................... 2,393
Last Used Block......................... 2
PL/SQL procedure successfully completed.
SQL> select header_file,header_block from dba_segments where segment_name ='DAODAO';
HEADER_FILE HEADER_BLOCK
----------- ------------
4 2387
---段头没有改变，说明对象并没有变化,说明shrink 只是对表对象进行操作，所以它也不需要额外一倍空间。
ps: move的操作速度远远快于shrink 操作 ，不是一般的快，不是一个数量级的，而且shrink 会产生大量的undo 和redo 操作。