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表裏的一行對於一個數據塊太大的情況有二種(一行在一個數據塊裏放不下)
第一種情況:
INSERT的時候，INSERT時候行的大小就超一個塊的大小。Oracle把這行的數據存儲在一連串的數據塊裏(Oracle Stores the data for the row in a chain of data blocks)，這種情況稱爲行鏈接(Row Chain)，一般不可避免(除非使用更大的數據塊)。
第二種情況:
UPDATE的時候，UPDATE之後導致行的大小超過一個塊的大小，Oracle將遷移這行數據(整行)到一個新的數據塊(假設更新的行在新的數據塊下放得下，否則這裏發生一次行鏈接)，在行遷移之前的地方有一個指向遷移後數據塊的指針，行的rowid不會改變。
--
行鏈接 |
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 |
---------------------------------------------------------- |
0
recursive calls |
0
db block gets |
2
consistent gets |
0
physical reads |
--
行遷移 |
SQL>
show parameter block_size |
NAME TYPE
VALUE |
------------------------------------
----------- ------------------------------ |
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 |
---------------------------------------------------------- |
0
recursive calls |
0
db block gets |
1
consistent gets |
0
physical 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 |
---------------------------------------------------------- |
0
recursive calls |
0
db block gets |
2
consistent gets |
0
physical reads |
--
檢查行遷移的記錄 |
SQL> truncate table chained_rows; |
Table truncated. |
SQL>
analyze table t01
list 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. |
--
消除行遷移 |
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 |
---------------------------------------------------------- |
0
recursive calls |
0
db block gets |
1
consistent gets |
0
physical reads |