24  recursivecalls
        0  db block gets
        7  consistent gets
        0  physical reads
        0  redo size
       318  bytessent via SQL*Net to client
       453  bytesreceived via SQL*Net from client
        1  SQL*Net roundtrips to/fromclient
        0  sorts (memory)
        0  sorts (disk)
        0  rows processed
可以看到查询这个空表读了7个内存块。然后我们向表中插入1000万条数据后再回滚：
insert into test_table select level from dual connect bylevel<=10000000;

10000000 rows created.

Elapsed: 00:00:58.38
ETL@RACTEST> rollback;

Rollback complete.

Elapsed: 00:00:01.15

ETL@RACTEST> set autotrace traceonlystatistics;
ETL@RACTEST> select * from test_table;

no rows selected

Elapsed: 00:00:00.14

Statistics
----------------------------------------------------------
       283  recursivecalls
        1  db block gets
     15463  consistentgets
        0  physical reads
       176  redosize
       318  bytessent via SQL*Net to client
       453  bytesreceived via SQL*Net from client
        1  SQL*Net roundtrips to/fromclient
        4  sorts (memory)
        0  sorts (disk)
        0  rows processed
可以看到，同样是读一个空表，但是却做了15463(consistent gets)+1(db blockgets)个逻辑读，可见delete操作不会修改表的高水标记。并且delete操作也很耗费时间，因此我们通常在想清空一个表的数据时用truncate来替代delete。truncate会以一种快速的方式清空表，只产生很少的日志信息，并且会将高水标记清零。例如：
ETL@RACTEST> insert into test_table select levelfrom dual connect by level<=10000000;

10000000 rows created.

Elapsed: 00:00:32.45
ETL@RACTEST> commit;

Commit complete.

Elapsed: 00:00:00.02
ETL@RACTEST> truncate table test_table;

Table truncated.

Elapsed: 00:00:29.52
ETL@RACTEST> set autotrace traceonlystatistics;
ETL@RACTEST> select * from test_table;

no rows selected

Elapsed: 00:00:00.00

Statistics
----------------------------------------------------------
        1  recursive calls
        1  db block gets
        6  consistent gets
        0  physical reads
       96  redosize
       318  bytessent via SQL*Net to client
       453  bytesreceived via SQL*Net from client
        1  SQL*Net roundtrips to/fromclient
        0  sorts (memory)
        0  sorts (disk)
        0  rows processed
truncate后再查询只做了7个逻辑读，也就是读了7个内存块。
但是truncate操作有一个限制，例如B表有一个外键参照A表的一个字段，那么truncate A表Oracle就会报错，实验如下：
create table ttt1 (a number,primary key(a));

Table created.

Elapsed: 00:00:00.02
ETL@RACTEST> insert into ttt1 select level from dualconnect by level<=10;

10 rows created.

Elapsed: 00:00:00.02
ETL@RACTEST> commit;

Commit complete.

Elapsed: 00:00:00.00
ETL@RACTEST> select * from ttt1;

        A
----------
        1
        2
        3
        4
        5
        6
        7
        8
        9
       10

10 rows selected.

Elapsed: 00:00:00.00
ETL@RACTEST> create table ttt2 (a number,constraintfk_ttt2_a foreign key(a) references ttt1(a));

Table created.
这时ttt2表的a字段参照了ttt1表的a字段，我如果用delete操作清空ttt1表的值是可以的，因为ttt2表没有数据，删除ttt1的数据不会违背外键参照完整性。
ETL@RACTEST> delete from ttt1;

10 rows deleted.
但是如果我们使用truncate,oracle则要报错：
ETL@RACTEST> truncate table ttt1;
truncate table ttt1
            *
ERROR at line 1:
ORA-02266: unique/primary keys in table referenced by enabledforeign keys

因为truncate实现的是一种快速删除，因此它无法检验参照完整性，即使删除的数据不违反参照完整性，Oracle也不允许做truncate操作，也就是我们说的宁可错杀一千，不可放过一个，呵呵。
我在培训的时候有人说外键如果设置了级联删除就可以truncate，但实际上是不行的，实验如下：
ETL@RACTEST> drop table ttt2 purge;

Table dropped.

Elapsed: 00:00:00.03
ETL@RACTEST> create table ttt2 (a number,constraintfk_ttt2_a foreign key(a) references ttt1(a) on deletecascade);

Table created.

Elapsed: 00:00:00.01
ETL@RACTEST> truncate table ttt1;
truncate table ttt1
            *
ERROR at line 1:
ORA-02266: unique/primary keys in table referenced by enabledforeign keys

Elapsed: 00:00:00.00
这是一种方式的级联删除，trucate仍然报错，下面是另一种级联：
ETL@RACTEST> drop table ttt2 purge;

Table dropped.

Elapsed: 00:00:00.01
ETL@RACTEST> create table ttt2 (a number,constraintfk_ttt2_a foreign key(a) references ttt1(a) on delete setnull);

Table created.

Elapsed: 00:00:00.01
ETL@RACTEST> truncate table ttt1;
truncate table ttt1
            *
ERROR at line 1:
ORA-02266: unique/primary keys in table referenced by enabledforeign keys
同样不行，因此得出结论，只要有外键参照A表的字段，那么就不能对A表使用truncate操作！