SET UNUSED列可以恢复吗？
问：
使用 SET UNUSED 选项可以标记一列或者多列不可用，对于SET UNUSED列可以恢复吗？
如果可以，如何恢复？
答：
首先我们了解一下SET UNUSED选项的功能和语法。
功能
使用 SET UNUSED 选项标记一列或者多列不可用， 设置UNUSED的作用是为了在cpu、内存等资源不充足的时候，先做上UNUSED标记再等数据库资源空闲的时候用DROP SET UNUSED删除。
语法
ALTER TABLE table SET UNUSED (多个列名，使用逗号隔开) 或 ALTER TABLE table SET UNUSED COLUMN colName;
ALTER TABLE table DROP UNUSED COLUMNS;
SET UNUSED列能否恢复？
设置 UNUSED 列之后，并不是将该列数据立即删除，而是被隐藏起来，物理上还是存在的，因此可以恢复，
但是恢复过程需要修改底层的数据字典并重启数据库，因此在执行 SET UNUSED操作时务必慎重！
恢复步骤
1.创建测试表
我们在scott用户下创建测试表hoegh，并插入4条数据。
点击(此处)折叠或打开
SCOTT@HOEGH>
SCOTT@HOEGH> create table hoegh(id number,name varchar2(20),dynasty varchar2(10));
Table created.
SCOTT@HOEGH> insert into hoegh values(1,'xishi','chunqiu');
1 row created.
SCOTT@HOEGH> insert into hoegh values(2,'wangzhaojun','xihan');
1 row created.
SCOTT@HOEGH> insert into hoegh values(3,'diaochan','donghan');
1 row created.
SCOTT@HOEGH> insert into hoegh values(4,'yangyuhuan','tang');
1 row created.
SCOTT@HOEGH> commit;
Commit complete.
SCOTT@HOEGH> desc hoegh
Name Null? Type
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME VARCHAR2(20)
AGE VARCHAR2(10)
SCOTT@HOEGH> select * from hoegh;
ID NAME DYNASTY
---------- -------------------- ----------
1 xishi chunqiu
2 wangzhaojun xihan
3 diaochan donghan
4 yangyuhuan tang
SCOTT@HOEGH>
2.设置并删除UNUSED列
我们选择hoegh表的第3列DYNASTY进行测试。
点击(此处)折叠或打开
SCOTT@HOEGH>
SCOTT@HOEGH> alter table hoegh set unused column DYNASTY;
Table altered.
SCOTT@HOEGH> desc hoegh
Name Null? Type
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME VARCHAR2(20)
SCOTT@HOEGH>
SCOTT@HOEGH> select * from hoegh;
ID NAME
---------- --------------------
1 xishi
2 wangzhaojun
3 diaochan
4 yangyuhuan
SCOTT@HOEGH>
3.根据测试表的OBJECT_ID修改数据字典信息
首先我们查询hoegh表的对象编号，然后切换到sys用户下，根据对象编号修改底层的数据字典信息。
点击(此处)折叠或打开
SCOTT@HOEGH>
SCOTT@HOEGH> SELECT OBJECT_ID FROM USER_OBJECTS
where OBJECT_NAME= 'HOEGH'; 2
OBJECT_ID
----------
102893
SCOTT@HOEGH>
SCOTT@HOEGH> conn / as sysdba
Connected.
SYS@HOEGH> select cols from tab$ where obj#=102893;
COLS
----------
2
SYS@HOEGH> update col$ set col#=intcol# where obj#=102893;
3 rows updated.
SYS@HOEGH>
SYS@HOEGH> update tab$ set cols=cols+1 where obj#=102893;
1 row updated.
SYS@HOEGH> update col$ set name='DYNASTY' where obj#=102893 and col#=3;
1 row updated.
SYS@HOEGH> update col$ set property=0 where obj#=102893;
3 rows updated.
SYS@HOEGH> commit;
Commit complete.
SYS@HOEGH>
SYS@HOEGH> desc scott.hoegh
Name Null? Type
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME VARCHAR2(20)
SYS@HOEGH>
4.重启数据库，确实测试表是否恢复
我们发现hoegh表的DYNASTY列仍然看不到，此时我们需要重启数据库。
点击(此处)折叠或打开
SYS@HOEGH>
SYS@HOEGH> shu immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SYS@HOEGH> startup
ORACLE instance started.
Total System Global Area 941600768 bytes
Fixed Size 1348860 bytes
Variable Size 717228804 bytes
Database Buffers 218103808 bytes
Redo Buffers 4919296 bytes
Database mounted.
Database opened.
SYS@HOEGH> desc scott.hoegh
Name Null? Type
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME VARCHAR2(20)
DYNASTY VARCHAR2(10)
SYS@HOEGH>
SYS@HOEGH> select * from scott.hoegh;
ID NAME DYNASTY
---------- -------------------- ----------
1 xishi chunqiu
2 wangzhaojun xihan
3 diaochan donghan
4 yangyuhuan tang
SYS@HOEGH>
SYS@HOEGH>
至此，恢复完成。
标记列为未使用 (Marking Columns Unused)
作者：dbtan |【转载时请以超链接形式标明文章出处和作者信息】
链接：http://www.dbtan.com/2016/12/marking-columns-unused.html
Marking Columns Unused 基本操作步骤
1. 标记列为未使用
SQL
ALTER TABLE <table_name> SET UNUSED (<column_name>);
2. 然后在数据库空闲时，再删除列
SQL
ALTER TABLE <table_name> DROP UNUSED COLUMNS CHECKPOINT <n>;
-- CHECKPOINT <n>
In the ALTER TABLE statement that follows, the optional clause CHECKPOINT is specified. This clause causes a checkpoint to be applied after processing the specified number of rows, in this case 250. Checkpointing cuts down on the amount of undo logs accumulated during the drop column operation to avoid a potential exhaustion of undo space.
ALTER TABLE hr.admin_emp DROP UNUSED COLUMNS CHECKPOINT 250;
参见官方文档：http://docs.oracle.com/cd/E11882_01/server.112/e25494/tables.htm#ADMIN11662
将列设为UNUSED时的系统行为
0. 实验环境 Oracle 11.2.0.4
SQL
17:03:36 sys@TQ(tq-78)> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production PL/SQL Release 11.2.0.4.0 - Production CORE 11.2.0.4.0 Production TNS for Linux: Version 11.2.0.4.0 - Production NLSRTL Version 11.2.0.4.0 - Production Elapsed: 00:00:00.00
1. 创建测试表 t18
SQL
17:04:18 tq@TQ(tq-78)> -- 创建表 t18 17:04:19 tq@TQ(tq-78)> create table t18 (a number, b number, c number, d number); Table created. Elapsed: 00:00:00.06
2. 查看表 t18 在数据库中分配的编号 object_id 为 152037
SQL
17:05:02 tq@TQ(tq-78)> -- 17:05:03 tq@TQ(tq-78)> SELECT object_id 17:05:03 2 FROM dba_objects 17:05:03 3 WHERE object_name = 'T18' 17:05:03 4 AND owner = 'TQ'; OBJECT_ID ---------- 152037 Elapsed: 00:00:00.01
3. 查看表 t18 在 col$ 数据字典中 col#, segcol#, intcol# 的值
SQL
17:05:48 tq@TQ(tq-78)> -- 17:05:49 tq@TQ(tq-78)> col name for a30; 17:05:49 tq@TQ(tq-78)> 17:05:49 tq@TQ(tq-78)> SELECT col#, segcol#, name, intcol#, type# 17:05:49 2 FROM sys.col$ 17:05:49 3 WHERE obj# IN (SELECT object_id 17:05:49 4 FROM dba_objects 17:05:49 5 WHERE object_name = 'T18' 17:05:49 6 AND owner = 'TQ'); COL# SEGCOL# NAME INTCOL# TYPE# ---------- ---------- ------------------------------ ---------- ---------- 1 1 A 1 2 2 2 B 2 2 3 3 C 3 2 4 4 D 4 2 Elapsed: 00:00:00.01
SQL
17:06:38 tq@TQ(tq-78)> -- 17:06:39 tq@TQ(tq-78)> col column_name for a20; 17:06:39 tq@TQ(tq-78)> col data_type for a20; 17:06:39 tq@TQ(tq-78)> 17:06:39 tq@TQ(tq-78)> select column_name, 17:06:39 2 data_type, 17:06:39 3 column_id, 17:06:39 4 hidden_column, 17:06:39 5 segment_column_id seg_cid, 17:06:39 6 internal_column_id internal_cid 17:06:39 7 from dba_tab_cols 17:06:39 8 where owner = 'TQ' 17:06:39 9 and table_name = 'T18'; COLUMN_NAME DATA_TYPE COLUMN_ID HID SEG_CID INTERNAL_CID -------------------- -------------------- ---------- --- ---------- ------------ A NUMBER 1 NO 1 1 B NUMBER 2 NO 2 2 C NUMBER 3 NO 3 3 D NUMBER 4 NO 4 4 Elapsed: 00:00:00.00
说明：col$ 数据字典中 col#, segcol#, intcol# 的意义：
COL# 可以表示该列是否在用（ 0 为 UNUSED ），
SEGCOL# 表示各列在数据块上存储时的顺序，
INTCOL# 表示创建表时各列的定义顺序。
4. 标记列 b,d 未使用，并查看表 t18 在 col$ 数据字典中 col#, segcol#, intcol# 的值
SQL
17:08:31 tq@TQ(tq-78)> -- 标记字段b,d 未使用 17:08:32 tq@TQ(tq-78)> alter table t18 set unused (b, d); Table altered. Elapsed: 00:00:00.17 17:08:35 tq@TQ(tq-78)> desc t18; Name Null? Type ----------------------------------------- -------- ---------------------------- A NUMBER C NUMBER
SQL
17:12:23 tq@TQ(tq-78)> -- 17:12:24 tq@TQ(tq-78)> col name for a30; 17:12:24 tq@TQ(tq-78)> 17:12:24 tq@TQ(tq-78)> SELECT col#, segcol#, name, intcol#, type# 17:12:24 2 FROM sys.col$ 17:12:24 3 WHERE obj# IN (SELECT object_id 17:12:24 4 FROM dba_objects 17:12:24 5 WHERE object_name = 'T18' 17:12:24 6 AND owner = 'TQ'); COL# SEGCOL# NAME INTCOL# TYPE# ---------- ---------- ------------------------------ ---------- ---------- 1 1 A 1 2 0 2 SYS_C00002_16121517:08:35$ 2 2 2 3 C 3 2 0 4 SYS_C00004_16121517:08:35$ 4 2 Elapsed: 00:00:00.00 17:12:24 tq@TQ(tq-78)> 17:12:24 tq@TQ(tq-78)> -- 17:12:24 tq@TQ(tq-78)> col column_name for a30; 17:12:24 tq@TQ(tq-78)> col data_type for a10; 17:12:24 tq@TQ(tq-78)> 17:12:24 tq@TQ(tq-78)> select column_name, 17:12:24 2 data_type, 17:12:24 3 column_id, 17:12:24 4 hidden_column, 17:12:24 5 segment_column_id seg_cid, 17:12:24 6 internal_column_id internal_cid 17:12:24 7 from dba_tab_cols 17:12:24 8 where owner = 'TQ' 17:12:24 9 and table_name = 'T18'; COLUMN_NAME DATA_TYPE COLUMN_ID HID SEG_CID INTERNAL_CID ------------------------------ ---------- ---------- --- ---------- ------------ A NUMBER 1 NO 1 1 SYS_C00002_16121517:08:35$ NUMBER YES 2 2 C NUMBER 2 NO 3 3 SYS_C00004_16121517:08:35$ NUMBER YES 4 4 Elapsed: 00:00:00.00
这里可以看到，在将列设为 UNUSED 之后，COL# 变为 0，其余的列的 COL# 重新排序。而此时该列在数据段上并没有被删除掉，因此其 SEGCOL# 列仍然保持原来的值。
这里原来的B,D列，其名字为系统自动生成的一列，命名形式为 SYS_CNNNNN_YYMMDDHH24:MI:SS$ ，NNNNN 为原来的COLUMN_ID，前面补0补足成5位数。hidden已经变为YES，COLUMN_ID为NULL。其他两列A,C的COLUMN_ID顺序作了调整。这三列的SEGMENT_COLUMN_ID和INTERNAL_COLUMN_ID没有变化。
SQL
17:20:47 tq@TQ(tq-78)> -- 在DBA_TAB_COLUMNS视图中，B,D列已经没有显示出来。 17:20:48 tq@TQ(tq-78)> select column_name, data_type, column_id 17:20:48 2 from dba_tab_columns 17:20:48 3 where owner = 'TQ' 17:20:48 4 and table_name = 'T18'; COLUMN_NAME DATA_TYPE COLUMN_ID ------------------------------ ---------- ---------- A NUMBER 1 C NUMBER 2 Elapsed: 00:00:00.00
5. 插入2条测试数据，并 DUMP 数据块
SQL
17:25:50 tq@TQ(tq-78)> insert into t18 values (1, 1); 1 row created. Elapsed: 00:00:00.02 17:25:52 tq@TQ(tq-78)> insert into t18 values (2, 2); 1 row created. Elapsed: 00:00:00.00 17:25:59 tq@TQ(tq-78)> commit; Commit complete. Elapsed: 00:00:00.00 17:26:01 tq@TQ(tq-78)> select dbms_rowid.rowid_relative_fno(rowid) file#,dbms_rowid.rowid_block_number(rowid) block#,a,c from t18; FILE# BLOCK# A C ---------- ---------- ---------- ---------- 35 133443 1 1 35 133443 2 2 Elapsed: 00:00:00.04 17:26:17 tq@TQ(tq-78)> alter system checkpoint; System altered. Elapsed: 00:00:00.08 17:29:26 tq@TQ(tq-78)> alter system dump datafile 35 block 133443; System altered. Elapsed: 00:00:00.09 17:29:47 tq@TQ(tq-78)> select value from v$diag_info where name like 'De%'; VALUE ------------------------------------------------------------ /app/oracle/diag/rdbms/tq/tq/trace/tq_ora_25965.trc Elapsed: 00:00:00.08
block_row_dump: tab 0, row 0, @0x3f8e tl: 10 fb: --H-FL-- lb: 0x1 cc: 3 col 0: [ 2] c1 02 col 1: *NULL* col 2: [ 2] c1 02 tab 0, row 1, @0x3f84 tl: 10 fb: --H-FL-- lb: 0x1 cc: 3 col 0: [ 2] c1 03 col 1: *NULL* col 2: [ 2] c1 03 end_of_block_dump
6. 增加字段e，并插入数据，再次 DUMP 数据块
SQL
17:46:38 tq@TQ(tq-78)> alter table t18 add (e number); Table altered. Elapsed: 00:00:00.03 17:46:40 tq@TQ(tq-78)> desc t18; Name Null? Type ----------------------------------------- -------- ---------------------------- A NUMBER C NUMBER E NUMBER 17:46:45 tq@TQ(tq-78)> -- 17:47:05 tq@TQ(tq-78)> col name for a30; 17:47:05 tq@TQ(tq-78)> 17:47:05 tq@TQ(tq-78)> SELECT col#, segcol#, name, intcol#, type# 17:47:05 2 FROM sys.col$ 17:47:05 3 WHERE obj# IN (SELECT object_id 17:47:05 4 FROM dba_objects 17:47:05 5 WHERE object_name = 'T18' 17:47:05 6 AND owner = 'TQ'); COL# SEGCOL# NAME INTCOL# TYPE# ---------- ---------- ------------------------------ ---------- ---------- 1 1 A 1 2 0 2 SYS_C00002_16121517:08:35$ 2 2 2 3 C 3 2 0 4 SYS_C00004_16121517:08:35$ 4 2 3 5 E 5 2 Elapsed: 00:00:00.00 17:47:05 tq@TQ(tq-78)> 17:47:05 tq@TQ(tq-78)> -- 17:47:05 tq@TQ(tq-78)> col column_name for a30; 17:47:05 tq@TQ(tq-78)> col data_type for a10; 17:47:05 tq@TQ(tq-78)> 17:47:05 tq@TQ(tq-78)> select column_name, 17:47:05 2 data_type, 17:47:05 3 column_id, 17:47:05 4 hidden_column, 17:47:05 5 segment_column_id seg_cid, 17:47:05 6 internal_column_id internal_cid 17:47:05 7 from dba_tab_cols 17:47:05 8 where owner = 'TQ' 17:47:05 9 and table_name = 'T18'; COLUMN_NAME DATA_TYPE COLUMN_ID HID SEG_CID INTERNAL_CID ------------------------------ ---------- ---------- --- ---------- ------------ A NUMBER 1 NO 1 1 SYS_C00002_16121517:08:35$ NUMBER YES 2 2 C NUMBER 2 NO 3 3 SYS_C00004_16121517:08:35$ NUMBER YES 4 4 E NUMBER 3 NO 5 5 Elapsed: 00:00:00.00 17:48:24 tq@TQ(tq-78)> update t18 set e = 1 where a = 1; 1 row updated. Elapsed: 00:00:00.01 17:48:26 tq@TQ(tq-78)> update t18 set e = 2 where a = 2; 1 row updated. Elapsed: 00:00:00.00 17:48:33 tq@TQ(tq-78)> commit; Commit complete. Elapsed: 00:00:00.00 17:51:32 tq@TQ(tq-78)> select dbms_rowid.rowid_relative_fno(rowid) file#,dbms_rowid.rowid_block_number(rowid) block#,a,c,e from t18; FILE# BLOCK# A C E ---------- ---------- ---------- ---------- ---------- 35 133443 1 1 1 35 133443 2 2 2 Elapsed: 00:00:00.01 17:51:43 tq@TQ(tq-78)> alter system checkpoint; System altered. Elapsed: 00:00:00.02 17:51:58 tq@TQ(tq-78)> alter system dump datafile 35 block 133443; System altered. Elapsed: 00:00:00.01 17:52:19 tq@TQ(tq-78)> alter system checkpoint; System altered. Elapsed: 00:00:00.02 17:52:57 tq@TQ(tq-78)> select value from v$diag_info where name like 'De%'; VALUE ------------------------------------------------------------ /app/oracle/diag/rdbms/tq/tq/trace/tq_ora_25965.trc Elapsed: 00:00:00.03
block_row_dump: tab 0, row 0, @0x3f76 tl: 14 fb: --H-FL-- lb: 0x2 cc: 5 col 0: [ 2] c1 02 col 1: *NULL* col 2: [ 2] c1 02 col 3: *NULL* col 4: [ 2] c1 02 tab 0, row 1, @0x3f68 tl: 14 fb: --H-FL-- lb: 0x2 cc: 5 col 0: [ 2] c1 03 col 1: *NULL* col 2: [ 2] c1 03 col 3: *NULL* col 4: [ 2] c1 03 end_of_block_dump
7. 删除字段e，会同时删除标记为未使用的字段
SQL
17:56:02 tq@TQ(tq-78)> alter table t18 drop (e); Table altered. Elapsed: 00:00:00.20 17:56:03 tq@TQ(tq-78)> desc t18; Name Null? Type ----------------------------------------- -------- ---------------------------- A NUMBER C NUMBER 17:56:10 tq@TQ(tq-78)> -- 17:56:26 tq@TQ(tq-78)> col name for a30; 17:56:26 tq@TQ(tq-78)> 17:56:26 tq@TQ(tq-78)> SELECT col#, segcol#, name, intcol#, type# 17:56:26 2 FROM sys.col$ 17:56:26 3 WHERE obj# IN (SELECT object_id 17:56:26 4 FROM dba_objects 17:56:26 5 WHERE object_name = 'T18' 17:56:26 6 AND owner = 'TQ'); COL# SEGCOL# NAME INTCOL# TYPE# ---------- ---------- ------------------------------ ---------- ---------- 1 1 A 1 2 2 2 C 2 2 Elapsed: 00:00:00.00 17:56:26 tq@TQ(tq-78)> 17:56:26 tq@TQ(tq-78)> -- 17:56:26 tq@TQ(tq-78)> col column_name for a30; 17:56:26 tq@TQ(tq-78)> col data_type for a10; 17:56:26 tq@TQ(tq-78)> 17:56:26 tq@TQ(tq-78)> select column_name, 17:56:26 2 data_type, 17:56:26 3 column_id, 17:56:26 4 hidden_column, 17:56:26 5 segment_column_id seg_cid, 17:56:26 6 internal_column_id internal_cid 17:56:26 7 from dba_tab_cols 17:56:26 8 where owner = 'TQ' 17:56:26 9 and table_name = 'T18'; COLUMN_NAME DATA_TYPE COLUMN_ID HID SEG_CID INTERNAL_CID ------------------------------ ---------- ---------- --- ---------- ------------ A NUMBER 1 NO 1 1 C NUMBER 2 NO 2 2 Elapsed: 00:00:00.01 17:56:27 tq@TQ(tq-78)> select dbms_rowid.rowid_relative_fno(rowid) file#,dbms_rowid.rowid_block_number(rowid) block#,a,c from t18; FILE# BLOCK# A C ---------- ---------- ---------- ---------- 35 133443 1 1 35 133443 2 2 Elapsed: 00:00:00.01 17:56:47 tq@TQ(tq-78)> alter system checkpoint; System altered. Elapsed: 00:00:00.02 17:56:52 tq@TQ(tq-78)> alter system dump datafile 35 block 133443; System altered. Elapsed: 00:00:00.01 17:57:00 tq@TQ(tq-78)> alter system checkpoint; System altered. Elapsed: 00:00:00.02 17:57:05 tq@TQ(tq-78)> select value from v$diag_info where name like 'De%'; VALUE ------------------------------------------------------------ /app/oracle/diag/rdbms/tq/tq/trace/tq_ora_25965.trc Elapsed: 00:00:00.00
block_row_dump: tab 0, row 0, @0x3f76 tl: 9 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 2] c1 02 col 1: [ 2] c1 02 tab 0, row 1, @0x3f68 tl: 9 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 2] c1 03 col 1: [ 2] c1 03 end_of_block_dump
可以看出B,D列和E列已经被删除。从这个实验就可以看出，在删除E时会将UNUSED列一并删除。
DUMP出数据块可以发现，块中每一行只有2列(即：A,C)。
总结：
因此 SET UNUSED 只是标记该列未使用，只是修改了数据字典，速度较快。
SET UNUSED (<column_name>) 比 DROP (<column_name>) 可以节省时间，因为只需修改数据字典不用动到数据块。
而DROP (<column_name>)，不仅修改数据字典，而且修改实际的块数据。如果表比较大，会耗费比较长的时间，会阻止其他dml的，所以一般都是在系统空闲下来的时候才做drop的。
如何恢复 SET UNUSED 的 COLUMN
通过上篇文章，我们已经基本了解 标记列为未使用 (Marking Columns Unused) 了。
我们知道，在 SET UNUSED 后 其实数据并未真的被删除，若这时又想恢复该列，有办法吗？
接下来，我们来实验一下。
如何修复被标记为 UNUSED 的字段
0. 实验环境 Oracle 11.2.0.4
SQL
16:49:06 sys@TQ(tq-78)> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production PL/SQL Release 11.2.0.4.0 - Production CORE 11.2.0.4.0 Production TNS for Linux: Version 11.2.0.4.0 - Production NLSRTL Version 11.2.0.4.0 - Production Elapsed: 00:00:00.00
1. 创建实验表 T20 ，插入测试数据，并将字段C 标记列为未使用
SQL
16:51:18 tq@TQ(tq-78)> -- 创建实验表 T20 16:51:18 tq@TQ(tq-78)> create table t20 (a number, b number, c varchar2(10), d number); Table created. Elapsed: 00:00:00.06 16:51:23 tq@TQ(tq-78)> -- 插入测试数据 16:51:24 tq@TQ(tq-78)> insert into t20 values (1, 2, '3', 4); 1 row created. Elapsed: 00:00:00.02 16:53:12 tq@TQ(tq-78)> insert into t20 values (2, 3, 'a', 5); 1 row created. Elapsed: 00:00:00.00 16:53:28 tq@TQ(tq-78)> insert into t20 values (3, 4, 'b', 6); 1 row created. Elapsed: 00:00:00.01 16:53:39 tq@TQ(tq-78)> commit; Commit complete. Elapsed: 00:00:00.00 16:58:35 tq@TQ(tq-78)> -- 将t20表中的C字段标记列为未使用 16:58:39 tq@TQ(tq-78)> ALTER TABLE t20 SET UNUSED COLUMN C; Table altered. Elapsed: 00:00:00.22 16:58:53 tq@TQ(tq-78)> desc t20; Name Null? Type ----------------------------------------- -------- ---------------------------- A NUMBER B NUMBER D NUMBER
2. 以下进行恢复 以管理员的身份登陆
2.1 查看表 t20 在数据库中分配的编号 object_id 为 152097
SQL
17:04:03 sys@TQ(tq-78)> -- 查看表 t20 在数据库中分配的编号 object_id 为 152097 17:04:03 sys@TQ(tq-78)> select object_id 17:04:03 2 from dba_objects 17:04:03 3 where object_name = 'T20' 17:04:03 4 and owner = 'TQ'; OBJECT_ID ---------- 152097 Elapsed: 00:00:00.01 17:07:32 sys@TQ(tq-78)> col name for a30; 17:07:52 sys@TQ(tq-78)> -- 17:08:02 sys@TQ(tq-78)> SELECT col#, segcol#, name, intcol#, type# 17:08:02 2 FROM sys.col$ 17:08:02 3 WHERE obj# = 152097; COL# SEGCOL# NAME INTCOL# TYPE# ---------- ---------- ------------------------------ ---------- ---------- 1 1 A 1 2 2 2 B 2 2 0 3 SYS_C00003_16121916:58:53$ 3 1 3 4 D 4 2 Elapsed: 00:00:00.00 17:11:05 sys@TQ(tq-78)> -- 字段数变为3了 17:11:06 sys@TQ(tq-78)> SELECT COLS FROM sys.TAB$ WHERE OBJ# = 152097; COLS ---------- 3 Elapsed: 00:00:00.01
2.2 更新相应的数据字典 sys.col$ 和 sys.tab$ 的值，并刷新 SHARED_POOL
SQL
17:20:51 sys@TQ(tq-78)> -- 17:20:59 sys@TQ(tq-78)> UPDATE sys.COL$ SET COL# = INTCOL# WHERE OBJ# = 152097; 4 rows updated. Elapsed: 00:00:00.01 17:21:01 sys@TQ(tq-78)> -- 17:21:17 sys@TQ(tq-78)> UPDATE sys.TAB$ SET COLS = COLS + 1 WHERE OBJ# = 152097; 1 row updated. Elapsed: 00:00:00.00 17:21:19 sys@TQ(tq-78)> -- 17:21:29 sys@TQ(tq-78)> UPDATE sys.COL$ 17:21:29 2 SET NAME = 'C' 17:21:29 3 WHERE OBJ# = 152097 17:21:29 4 AND COL# = 3; 1 row updated. Elapsed: 00:00:00.00 17:21:30 sys@TQ(tq-78)> -- 17:21:40 sys@TQ(tq-78)> UPDATE sys.COL$ SET PROPERTY = 0 WHERE OBJ# = 152097; 4 rows updated. Elapsed: 00:00:00.00 17:21:41 sys@TQ(tq-78)> commit; Commit complete. Elapsed: 00:00:00.01 17:24:33 sys@TQ(tq-78)> -- 刷新 SHARED_POOL 17:26:26 sys@TQ(tq-78)> alter system flush SHARED_POOL; System altered. Elapsed: 00:00:00.11
2.3 验证 t20 表的字段C 已经恢复完成
SQL
17:28:40 tq@TQ(tq-78)> desc t20; Name Null? Type ----------------------------------------- -------- ---------------------------- A NUMBER B NUMBER C VARCHAR2(10) D NUMBER 17:29:07 tq@TQ(tq-78)> select * from t20; A B C D ---------- ---------- ---------- ---------- 1 2 3 4 2 3 a 5 3 4 b 6 Elapsed: 00:00:00.00
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