关于alter table move
alter table move主要有两方面的作用
1.用来移动table到其他表空间
2.用来减少table中的存储碎片，优化存储空间和性能
案例1：移动table到其他表空间
--alter table move
SQL> create table t tablespace system as select * from all_objects;
Table created.
SQL> select table_name,tablespace_name from user_tables where table_name='T';
TABLE_NAME TABLESPACE_NAME
------------------------------ ------------------------------
T SYSTEM
在表上建立一个索引
SQL> create index idx_t_object_id on t(object_id);
Index created.
SQL> select index_name,status from user_indexes where table_name='T';
INDEX_NAME STATUS
------------------------------ --------
IDX_T_OBJECT_ID
当移动表到指定表空间时，用户在目标表空间必须有足够的表空间配额，否则会报错，比如
ERROR at line 1:
ORA-01950: no privileges on tablespace 'SYSTEM' SQL> alter table t move tablespace tbs1;
Table altered.
移动到其他表空间后，索引就会失效。因为索引是通过rowid来定位记录的，当table迁移到其他表空间，或者进行
alter table ...move清理碎片的时候，索引失效。
SQL> select index_name,status from user_indexes where table_name='T';
INDEX_NAME STATUS
------------------------------ --------
IDX_T_OBJECT_ID UNUSABLE
重建索引
SQL> alter index idx_t_object_id rebuild tablespace tbs1;
Index altered.
SQL> select index_name,status from user_indexes where table_name='T';
INDEX_NAME STATUS
------------------------------ --------
IDX_T_OBJECT_ID VALID
else：alter table...move语句会在目标对象上放置X锁，如果需要移动的范围比较大，将会花费比较长的时间
X锁的时间也会比较长。对于其他会话需要等到该X锁释放后才能执行该语句
2 employee_id number(6),
3 last_name varchar2(20),
4 first_name varchar2(25),
5 constraint t_emp_pk primary key(employee_id))
6 organization index;
Table created.
SQL> insert into t_emp select employee_id,last_name,first_name
2 from hr.employees;
108 rows created.
SQL> commit;
Commit complete.
SQL> alter table t_emp move tablespace users online;
Table altered.
案例2：减少table存储碎片，优化性能
Table created.
SQL> exec dbms_stats.gather_table_stats(user,'T');
PL/SQL procedure successfully completed.
SQL> exec show_space('T'); --此处使用tom的show_space过程来查看表的存储情况
Unformatted Blocks ..................... 0
FS1 Blocks (0-25) ..................... 0
FS2 Blocks (25-50) ..................... 0
FS3 Blocks (50-75) ..................... 0
FS4 Blocks (75-100)..................... 0
Full Blocks ..................... 1,025
Total Blocks............................ 1,152
Total Bytes............................. 9,437,184
Total MBytes............................ 9
Unused Blocks........................... 101
Unused Bytes............................ 827,392
Last Used Ext FileId.................... 4
Last Used Ext BlockId................... 1,280
Last Used Block......................... 27
PL/SQL procedure successfully completed.
其中total blocks表示分配给表的总的block数，unusedblock表示位于高水位线以上的从未使用
过的block
SQL> select segment_name,bytes from user_extents where segment_name='T';
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
1048576
T
1048576
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
1048576
T
1048576
T
1048576
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
1048576
T
1048576
T
1048576
24 rows selected.
此时T表有25个区的存储空间
删除T表的部分数据，验证存储空间变化
SQL> delete from t where object_id<35000;
34221 rows deleted.
SQL> commit;
Commit complete.
SQL> exec show_space('T');
Unformatted Blocks ..................... 0
FS1 Blocks (0-25) ..................... 0
FS2 Blocks (25-50) ..................... 1
FS3 Blocks (50-75) ..................... 0
FS4 Blocks (75-100)..................... 479
Full Blocks ..................... 545
Total Blocks............................ 1,152
Total Bytes............................. 9,437,184
Total MBytes............................ 9
Unused Blocks........................... 101
Unused Bytes............................ 827,392
Last Used Ext FileId.................... 4
Last Used Ext BlockId................... 2,560
Last Used Block......................... 27
PL/SQL procedure successfully completed.
相较于delete操作之前，并没有发生变化，说明此时表T已经存在存储空间碎片的情况，下面使用
alter table move，这里需要注意的一点是：
alter table move 操作来清理存储碎片，其实是读取t,然后在t表所在的表空间重建表t.等moce
操作完成之后删除原来的表。所以需要额外的存储空间开销，要求当前表空间的大小至少为
表的两倍
SQL> alter table t move;
Table altered.
SQL> exec show_space('T');
Unformatted Blocks ..................... 0
FS1 Blocks (0-25) ..................... 0
FS2 Blocks (25-50) ..................... 0
FS3 Blocks (50-75) ..................... 0
FS4 Blocks (75-100)..................... 0
Full Blocks ..................... 545
Total Blocks............................ 640
Total Bytes............................. 5,242,880
Total MBytes............................ 5
Unused Blocks........................... 77
Unused Bytes............................ 630,784
Last Used Ext FileId.................... 4
Last Used Ext BlockId................... 3,072
Last Used Block......................... 51
PL/SQL procedure successfully completed.
这个结果可以看出,hwm降下来了，并且还回收了一部分分配给表的block。
SQL> select segment_name,bytes from user_extents where segment_name='T';
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
65536
T
65536
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
65536
T
1048576
T
1048576
SEGMENT_NAME
--------------------------------------------------------------------------------
BYTES
----------
T
1048576
T
1048576
20 rows selected.
相比较于之前25个区的存储空间，delete之后只剩了20个区的空间，所以在alter table move 的过程中
也会释放分配给表的block