Oracle插入记录的顺序是否是读取的顺序？
通过一个简单的实验验证：
SQL> create table t
( x int,
a char(2000) default 'x',
b char(2000) default 'x',
c char(2000) default 'x');
Table created.
SQL> insert into t (x) values ( 1 );
1 row created.
SQL> insert into t (x) values ( 2);
1 row created.
SQL> insert into t (x) values ( 3);
1 row created.
SQL> commit;
Commit complete.
SQL> select x, rownum, rowid from t;
X ROWNUM ROWID
---------- ---------- ------------------
3 1 AAAOXNAAHAAAAasAAA
1 2 AAAOXNAAHAAAAavAAA
2 3 AAAOXNAAHAAAAawAAA
SQL> delete from t where x = 2;
1 row deleted.
SQL> commit;
Commit complete.
SQL> select x, rownum, rowid from t;
X ROWNUM ROWID
---------- ---------- ------------------
3 1 AAAOXNAAHAAAAasAAA
1 2 AAAOXNAAHAAAAavAAA
SQL> insert into t (x) values ( 4 );
1 row created.
SQL> select x, rownum, rowid from t;
X ROWNUM ROWID
---------- ---------- ------------------
3 1 AAAOXNAAHAAAAasAAA
1 2 AAAOXNAAHAAAAavAAA
4 3 AAAOXNAAHAAAAawAAA
insert into t (x) values ( 5);
SQL> select x, rownum, rowid from t;
X ROWNUM ROWID
---------- ---------- ------------------
3 1 AAAOXNAAHAAAAasAAA
5 2 AAAOXNAAHAAAAatAAA
1 3 AAAOXNAAHAAAAavAAA
4 4 AAAOXNAAHAAAAawAAA
SQL> insert into t (x) values ( 10);
1 row created.
SQL> select x, rownum, rowid from t;
X ROWNUM ROWID
---------- ---------- ------------------
3 1 AAAOXNAAHAAAAasAAA
5 2 AAAOXNAAHAAAAatAAA
10 3 AAAOXNAAHAAAAauAAA
1 4 AAAOXNAAHAAAAavAAA
4 5 AAAOXNAAHAAAAawAAA
可见Oracle读取时按照记录的ROWID默认升序排列的，Oracle是一种堆表(默认)，堆的意思就是杂乱无章的，插入数据时是根据内部算法，找到可用的数据块，一般出于效率的考虑，不采用原来的空间，用逻辑块的新空间，读取的顺序与COMMIT也没有直接关系，所以要排序，最好用ORDER BY。
