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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。
--
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 |
---------------------------------------------------------- |
0
recursive calls |
0
db block gets |
2
consistent gets |
0
physical reads |
--
行迁移 |
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 |
--
检查行迁移的记录 |
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 |
---------------------------------------------------------- |
0
recursive calls |
0
db block gets |
1
consistent gets |
0
physical reads |