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set unused column column_name 和…

2017-05-02 10:48 288 查看

http://hi.baidu.com/dba_jungle/blog/item/e0e0fa3ed39db00dbaa1671d.html
要删除表中不用的字段,如果直接drop
column,对于大表,进行DDL操作的时间会比较长,会严重阻塞DML语句,导致应用服务器crash,通常的做法是先set
unused column column_name 或者 set unused
column_name,column_name...,然后再利用停机时间做alter table table_name drop
unused columns。
set unused column是对列进行逻辑删除，而drop
column是对列进行物理删除，它们的共同点是对列进行删除，使用set unused
column删除列更快一些，逻辑删除好处就是可以当资源紧张的时候，节约处理时间，当以后空余的时间再真正DROP掉。

SQL> select * from test
where rownum<10;

NICK
ID

-------------------- ----------

zhaolin1346
3

zhaolin1347
3

zhaolin1348
3

zhaolin1349
3

zhaolin1350
3

zhaolin1351
3

zhaolin1352
3

zhaolin1353
3

zhaolin1354
3

9 rows selected.

SQL> create
index idx_test_id on test(id);

Index created.

SQL> select
index_name,table_name from user_indexes where
table_name='TEST';

INDEX_NAME
TABLE_NAME

------------------------------ ------------------------------

IDX_TEST_NICK
TEST

IDX_TEST_ID
TEST

SQL> ALTER
TABLE TEST SET UNUSED COLUMN ID;

Table altered.

SQL> select
index_name,table_name from user_indexes where
table_name='TEST';

INDEX_NAME
TABLE_NAME

------------------------------ ------------------------------

IDX_TEST_NICK
TEST

SQL> desc
test;

Name
Null?
Type

----------------------------------------- --------
----------------------------

NICK
VARCHAR2(20)

SQL> alter
table test drop unused columns;

Table altered.

SQL> desc
test;

Name
Null?
Type

----------------------------------------- --------
----------------------------

NICK
4000

VARCHAR2(20)

另外,set unused column
column_name包含此列的索引也会被drop掉

--下面的附加实验是验证在set unused column
之后,在下次插入新的记录时,会不会再为此column分配存储空间

SQL> select
rowid,t.* from test t;

ROWID
NICK
ID

------------------ -------------------- ----------

AAAB6sAAFAAAAAuAAA
zhaolin
10

SQL>
select dbms_rowid.ROWID_RELATIVE_FNO(rowid) as file#,

2
dbms_rowid.ROWID_BLOCK_NUMBER(rowid) as block# from
test;


FILE#
BLOCK#

---------- ----------

5
46

SQL> alter system dump datafile 5 block
46;

System altered.

--以下是dump文件的信息

data_block_dump,data header at 0x5374464

===============

tsiz: 0x1f98

hsiz: 0x14

pbl: 0x05374464

bdba: 0x0140002e

76543210

flag=--------

ntab=1

nrow=1

frre=-1

fsbo=0x14

fseo=0x1f8a

avsp=0x1f76

tosp=0x1f76

0xe:pti[0] nrow=1 offs=0

0x12:pri[0] offs=0x1f8a

block_row_dump:

tab 0, row 0, @0x1f8a

tl: 14 fb: --H-FL-- lb: 0x1 cc: 2

col 0: [ 7] 7a 68 61 6f 6c 69
6e
col 1: [
2] c1 0b
end_of_block_dump

End dump data blocks tsn: 7 file#: 5 minblk 46 maxblk 46

--在set
unused column
id之后，对于新插入的记录，id列会不会不用再分配存储空间了
SQL>
alter table test set unused column id;

Table altered.

SQL> insert
into test(nick) values('nature');

1 row created.

SQL>
commit;

Commit complete.

SQL> select
dbms_rowid.ROWID_RELATIVE_FNO(rowid) as file#,

2
dbms_rowid.ROWID_BLOCK_NUMBER(rowid) as block# from
test;


FILE#
BLOCK#

---------- ----------

5
46

5
46

SQL> alter
system dump datafile 5 block 46;

System altered.

SQL> select *
from test;

NICK

--------------------

zhaolin

nature

--以下是dump文件的信息

data_block_dump,data header at 0x5374464

===============

tsiz: 0x1f98

hsiz: 0x16

pbl: 0x05374464

bdba: 0x0140002e

76543210

flag=--------

ntab=1

nrow=2

frre=-1

fsbo=0x16

fseo=0x1f7f

avsp=0x1f69

tosp=0x1f69

0xe:pti[0] nrow=2 offs=0

0x12:pri[0] offs=0x1f8a

0x14:pri[1] offs=0x1f7f

block_row_dump:

tab 0, row 0, @0x1f8a

tl: 14 fb: --H-FL-- lb: 0x1 cc: 2

col 0: [ 7] 7a 68 61 6f 6c 69
6e
col 1: [
2] c1 0b --原来的空间并不会释放
tab 0, row 1,
@0x1f7f

tl: 11 fb: --H-FL-- lb: 0x2 cc: 2

col 0: [ 6] 6e 61 74 75 72
65
col 1: [0]
--从这里可以看出，不会再分配存储空间了,但是保留着列的标识符
end_of_block_dump

End dump data blocks tsn: 7 file#: 5 minblk 46 maxblk 46

我们新增加列是定长型的列address
char(10)
SQL> alter table test
add(address char(10));

Table altered.

SQL> select
dbms_rowid.ROWID_RELATIVE_FNO(rowid) as file#,

2
dbms_rowid.ROWID_BLOCK_NUMBER(rowid) as block# from
test;


FILE#
BLOCK#

---------- ----------

5
46

5
46

SQL> alter system dump
datafile 5 block 46;

System altered.

SQL> desc
test;

Name
Null?
Type

----------------------------------------- --------
----------------------------

NICK
VARCHAR2(20)

ADDRESS
CHAR(10)

SQL> insert
into test values('cat','china hang');

1 row created.

SQL>
commit;

Commit complete.

SQL> alter system dump
datafile 5 block 46;

System altered.

SQL> select
dbms_rowid.ROWID_RELATIVE_FNO(rowid) as file#,

2
dbms_rowid.ROWID_BLOCK_NUMBER(rowid) as block# from
test;


FILE#
BLOCK#

---------- ----------

5
46

5
46

5
46

--dump文件如下

data_block_dump,data header at 0x5374464

===============

tsiz: 0x1f98

hsiz: 0x1a

pbl: 0x05374464

bdba: 0x0140002e

76543210

flag=--------

ntab=1

nrow=4

frre=-1

fsbo=0x1a

fseo=0x1f62

avsp=0x1f48

tosp=0x1f48

0xe:pti[0] nrow=4 offs=0

0x12:pri[0] offs=0x1f8a

0x14:pri[1] offs=0x1f7f

0x16:pri[2] offs=0x1f6c

0x18:pri[3] offs=0x1f62

block_row_dump:

tab 0, row 0, @0x1f8a

tl: 14 fb: --H-FL-- lb: 0x0 cc: 2

col 0: [ 7] 7a 68 61 6f 6c 69
6e

col 1: [ 2] c1 0b

tab 0, row 1, @0x1f7f

tl: 11 fb: --H-FL-- lb: 0x0 cc: 2

col 0: [ 6] 6e 61 74 75 72
65

col 1: [0]

tab 0, row 2, @0x1f6c

tl: 19 fb: --H-FL-- lb: 0x1 cc: 3

col 0: [ 3] 63 61 74

col 1: [0]
col 2:
[10] 63 68 69 6e 61 20 68 61 6e 67

tab 0, row 3, @0x1f62

tl: 10 fb: --H-FL-- lb: 0x2 cc: 3

col 0: [ 4] 6c 61 73 74

col 1: [0]
col 2: [0]
--从这里可以看出，不会再分配存储空间了

end_of_block_dump

End dump data blocks tsn: 7 file#: 5 minblk 46 maxblk 46

所以在set unused
column之后,不会再为新插入的记录分配存储空间,要是在set unused
column之前新增加的那列是定长型,并且有默认值,还有NOT NULL约束,情况又是怎么样呢?

经过实验,也不会再分配存储空间了!

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