oracle 物理读,逻辑读的理解

一.物理读(physical read)
当数据块第一次读取到,就会缓存到buffer cache 中,而第二次读取和修改该数据块时就在内存buffer cache 了 以下是例子:
1.1 第一次读取:
C:"Documents and Settings"Paul Yi>sqlplus "/as sysdba"
SQL*Plus: Release 9.2.0.4.0 - Production on Thu Feb 28 09:32:04 2008
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.4.0 - Production
SQL> set autotrace traceonly
SQL> select * from test;
Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=2 Card=4 Bytes=8)
   1    0   TABLE ACCESS (FULL) OF 'TEST' (Cost=2 Card=4 Bytes=8)
Statistics
----------------------------------------------------------
        175 recursive calls
          0 db block gets
         24 consistent gets
         9 physical reads             --9个物理读
          0 redo size
        373 bytes sent via SQL*Net to client
        503 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          2 sorts (memory)
          0 sorts (disk)
          1 rows processed
1.2 第二次读取
SQL> select * from test;
Execution Plan
----------------------------------------------------------
   0   &nbs