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行迁移与行链接问题解决实例

2014-11-21 17:34 453 查看
表里的一行对于一个数据块太大的情况有二种(一行在一个数据块里放不下)

第一种情况:

INSERT的时候，INSERT时候行的大小就超一个块的大小。Oracle把这行的数据存储在一连串的数据块里(Oracle Stores the data for the row in a chain of data blocks)，这种情况称为行链接(Row Chain)，一般不可避免(除非使用更大的数据块)。

第二种情况:

UPDATE的时候，UPDATE之后导致行的大小超过一个块的大小，Oracle将迁移这行数据(整行)到一个新的数据块(假设更新的行在新的数据块下放得下，否则这里发生一次行链接)，在行迁移之前的地方有一个指向迁移后数据块的指针，行的rowid不会改变。

view source

-- 行链接
SQL>
create

table
t01
2(
3 col01 varchar2(4000),
4 col02 varchar2(4000),
5 col03 varchar2(2000)
6)
7/
Table
 
created.
SQL>
insert

into
 
t01
values
(lpad(
'*'
,4000,
'*'
),lpad(
'*'
,4000,
'*'
),lpad(
'*'
,2000,
'*'
));
1 row created.
SQL>
commit
;
Commit
 
complete.
SQL>
select

rowid
from
 
t01;
ROWID
------------------
AAASpTAAEAAAALdAAA
SQL>
select

*
from
 

t01
where
 
rowid=
'AAASpTAAEAAAALdAAA'
;
Execution Plan
----------------------------------------------------------
Plan hash value: 2326159110
-----------------------------------------------------------------------------------
|Id|Operation|
Name
 
|

Rows
|Bytes |Cost (%CPU)|
Time
 
|
-----------------------------------------------------------------------------------
|0 |
SELECT

STATEMENT ||1 |5018 |1 (0)|00:00:01 |
|1 |
TABLE

ACCESS
BY
 
USER
 
ROWID|T01|1 |5018 |1 (0)|00:00:01 |
-----------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0recursive calls
0db block gets
2consistent gets
0physical reads
-- 行迁移
SQL>show parameter block_size
NAME

TYPEVALUE
------------------------------------ ----------- ------------------------------
db_block_size
integer
 
8192
SQL>
create

table
 
t01
2(
3 col01 varchar2(4000),
4 col02 varchar2(4000),
5 col03 varchar2(2000)
6)
7/
Table
 
created.
SQL>
insert

into
 
t01
2

values
(lpad(
'*'
,3000,
'*'
),lpad(
'*'
,3000,
'*'
),lpad(
'*'
,1000,
'*'
));
1 row created.
SQL>
insert

into
 
t01
2

values
(
'a'
,
'a'
,
'a'
);
1 row created.
SQL>
commit
;
Commit
 
complete.
SQL>
select

rowid
from
 

t01
where
 
col01=
'a'
;
ROWID
------------------
AAASpWAAEAAAALrAAB
SQL>
select

*
from
 

t01
where
 
rowid=
'AAASpWAAEAAAALrAAB'
;
Execution Plan
----------------------------------------------------------
Plan hash value: 2326159110
-----------------------------------------------------------------------------------
|Id|Operation|
Name
 
|

Rows
|Bytes |Cost (%CPU)|
Time
 
|
-----------------------------------------------------------------------------------
|0 |
SELECT

STATEMENT ||1 |5018 |1 (0)|00:00:01 |
|1 |
TABLE

ACCESS
BY
 
USER
 
ROWID|T01|1 |5018 |1 (0)|00:00:01 |
-----------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0recursive calls
0db block gets
1consistent gets
0physical reads
SQL>
update

t01
2
set
 
col01 =lpad(col01, 3000,
'a'
),
3 col02 = lpad(col02, 3000,
'a'
),
4 col03 = lpad(col03, 1000,
'a'
)
5

where
 
col01 =

'a'
;
1 row updated.
SQL>
commit
;
Commit
 
complete.
SQL>
select

rowid
from
 

t01
where
 
col01
like
 
'a%'
;
ROWID
------------------
AAASpWAAEAAAALrAAB
SQL>
select

rowid
from
 

t01
where
 
col01
like
 
'a%'
;
ROWID
------------------
AAASpWAAEAAAALrAAB
SQL>
set

autot trace
SQL>
select

*
from
 

t01
where
 
rowid=
'AAASpWAAEAAAALrAAB'
;
Execution Plan
----------------------------------------------------------
Plan hash value: 2326159110
-----------------------------------------------------------------------------------
|Id|Operation|
Name
 
|

Rows
|Bytes |Cost (%CPU)|
Time
 
|
-----------------------------------------------------------------------------------
|0 |
SELECT

STATEMENT ||1 |5018 |1 (0)|00:00:01 |
|1 |
TABLE

ACCESS
BY
 
USER
 
ROWID|T01|1 |5018 |1 (0)|00:00:01 |
-----------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0recursive calls
0db block gets
2consistent gets
0physical reads
-- 检查行迁移的记录
SQL>
truncate

table
 
chained_rows;
Table
 
truncated.
SQL>analyze 
table

t01list chained 
rows

into
 
chained_rows;
Table
 
analyzed.
SQL>
exec

print_table(
'select *from chained_rows'
)
OWNER_NAME: YAO
TABLE_NAME: T01
CLUSTER_NAME:
PARTITION_NAME:
SUBPARTITION_NAME : N/A
HEAD_ROWID: AAASpWAAEAAAALrAAB
ANALYZE_TIMESTAMP : 2012-12-24 11:31:51
-----------------
PL/SQL
procedure

successfully completed.
-- 消除行迁移
SQL>
alter

table
 
t01
move
;
Table
 
altered.
SQL>
select

rowid
from
 

t01
where
 
col01
like
 
'a%'
;
ROWID
------------------
AAASpZAAEAAAAL8AAA
SQL>
set

autot trace
SQL>
select

*
from
 

t01
where
 
rowid=
'AAASpZAAEAAAAL8AAA'
;
Execution Plan
----------------------------------------------------------
Plan hash value: 2326159110
-----------------------------------------------------------------------------------
|Id|Operation|
Name
 
|

Rows
|Bytes |Cost (%CPU)|
Time
 
|
-----------------------------------------------------------------------------------
|0 |
SELECT

STATEMENT ||1 |5018 |1 (0)|00:00:01 |
|1 |
TABLE

ACCESS
BY
 
USER
 
ROWID|T01|1 |5018 |1 (0)|00:00:01 |
-----------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0recursive calls
0db block gets
1consistent gets
0physical reads

                                            

                                        

                        
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