物理读与逻辑读

1、物理读即是把数据从磁盘读入到buffer catch的过程。

    通常情况下是，如果需要数据的时候发现不存在于buffer catch当中，即oracle就会执行物理读。
　例子如下：
C:/Documents and Settings/Administrator>sqlplus jenny/jenny
SQL> set autotrace traceonly;
SQL> select * from t_test1;
----------------------------------------------------------
Plan hash value: 1883417357
----------------------------------------------------------
0  recursive calls
0  db block gets
11  consistent gets
3  physical reads --发生了3次物理读写
0  redo size
10632  bytes sent via SQL*Net to client
451  bytes received via SQL*Net from client
8  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
97  rows processed

　SQL> select * from t_test1;
　----------------------------------------------------------
　Plan hash value: 1883417357
----------------------------------------------------------
0  recursive calls
0  db block gets
11  consistent gets
0  physical reads　--这里没有发生物理读取，因为这时数据已存在buffer catche中
0  redo size
10632  bytes sent via SQL*Net to client
451  bytes received via SQL*Net from client
8  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
97  row