创建一个表时表中列的顺序对性能的影响

create test as select * from dba_objects;
select header_file,header_block from 
dba_segments where owner='TEST' and segment_name='TEST';

HEADER_FILEHEADER_BLOCK
------------------------ ---------------------------
 131179
alter system dump datafile 13 block 1180

block_row_dump:
tab 0, row 0, @0x1f20
tl: 96 fb: --H-FL-- lb: 0x0cc: 13--------row header信息.
col0: [ 3]53 59 53-------------------COLUMN DATA
fb: --H-FL-- 是FLAG BYTE.
fb Flag Byte:
K = Cluster Key (Flags may change meaning if this is set to show HASH cluster)
C = Cluster table member
H = Head piece of row
D = Deleted row
F = First data piece
L = Last data piece
P = First column continues from previous piece
N = Last column continues in next piece
lb: 0x0-----------LOCK BYTE,锁信息
cc: 13------------COLUMN COUNT
col0---------第一列
[ 3]-------------COLUMN LENGTH
53 59 53---------实际数据

SQL> create table small (
2n0 number,
3n1 number,
4n2 number,
5n3 number,
6n4 number,
7n5 number,
8n6 number,
9n7 number,
 10n8 number,
 11n9 number
 12) pctfree 0;

Table created.

SQL> begin
2for i in 1..78 loop
3insert into small values (0,0,0,0,0,0,0,0,0,0);
4end loop;
5end;
6/

PL/SQL procedure successfully completed.

SQL> set timing on
SQL> declare
2n number;
3begin
4for i in 1..1000000 loop
5select sum(n0) into n from small;
6end loop;
7end;
8/

PL/SQL procedure successfully completed.

Elapsed: 00:07:437.30
SQL> declare
2n number;
3begin
4for i in 1..1000000 loop
5select sum(n9) into n from small;
6end loop;
7end;
8/

PL/SQL procedure successfully completed.

Elapsed: 00:08:482.13

SQL> create table null_order (
2column1 number,
3column2 number,
4column3 number
5);

Table created.

SQL> insert into null_order (column2) values (0);

1 row created.

SQL> select header_file, header_block from dba_segments
2where segment_name = 'TEST' and owner = 'TEST';

HEADER_FILE HEADER_BLOCK
----------- ------------
350010

SQL> alter system dump datafile 3 block 50011;

System altered.

block_row_dump:
tab 0, row 0, @0x7b2
tl: 6 fb: --H-FL-- lb: 0x1 cc: 2
col0: *NULL*--------------第一列的NULL
col1: [ 1]80----------------第二列的值(后面没有了第三列的值)
end_of_block_dump