1.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 SELECT STATEMENT ptimizer=CHOOSE (Cost=2 Card=4 Bytes=8)
1 0 TABLE ACCESS (FULL) OF 'TEST' (Cost=2 Card=4 Bytes=8)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
7 consistent gets
0 physical reads --没有发生物理读了,直接从buffer cache 中读取了
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
0 sorts (memory)
0 sorts (disk)
1 rows processed
1.3 数据块被重新读入buffer cache ,这种发生在
如果有新的数据需要被读入Buffer Cache中，而Buffer Cache又没有足够的空闲空间，Oracle就根据LRU算法将LRU链表中LRU端的数据置换出去。当这些数据被再次访问到时，需要重新从磁盘读入。
SQL> alter session set events 'immediate trace name flush_cache';--清空数据缓冲区
Session altered.
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
----------------------------------------------------------
0 recursive calls
0 db block gets
7 consistent gets
6 physical reads --又重新发生了物理读
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
0 sorts (memory)
0 sorts (disk)
1 rows processed
2.逻辑读(buffer read)
逻辑读指的就是从（或者视图从）Buffer Cache中读取数据块。按照访问数据块的模式不同，可以分为即时读（Current Read）和一致性读（Consistent Read）。注意：逻辑IO只有逻辑读，没有逻辑写。
- 即时读
即时读即读取数据块当前的最新数据。任何时候在Buffer Cache中都只有一份当前数据块。即时读通常发生在对数据进行修改、删除操作时。这时，进程会给数据加上行级锁，并且标识数据为“脏”数据。
SQL> select * from test for update;
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT ptimizer=CHOOSE (Cost=2 Card=4 Bytes=8)
1 0 FOR UPDATE
2 1 TABLE ACCESS (FULL) OF 'TEST' (Cost=2 Card=4 Bytes=8)
Statistics
----------------------------------------------------------
0 recursive calls
1 db block gets
14 consistent gets
0 physical reads
252 redo size
386 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>
- 一致性读
Oracle是一个多用户系统。当一个会话开始读取数据还未结束读取之前，可能会有其他会话修改它将要读取的数据。如果会话读取到修改后的数据，就会造成数据的不一致。一致性读就是为了保证数据的一致性。在Buffer Cache中的数据块上都会有最后一次修改数据块时的SCN。如果一个事务需要修改数据块中数据，会先在回滚段中保存一份修改前数据和SCN的数据块，然后再更新Buffer Cache中的数据块的数据及其SCN，并标识其为“脏”数据。当其他进程读取数据块时，会先比较数据块上的SCN和自己的SCN。如果数据块上的SCN小于等于进程本身的SCN，则直接读取数据块上的数据；如果数据块上的SCN大于进程本身的SCN，则会从回滚段中找出修改前的数据块读取数据。通常，普通查询都是一致性读。
下面这个例子帮助大家理解一下一致性读：
会话1中：
SQL> select * from test;
ID
----------
1000
SQL> update test set id=2000;
1 row updated.
会话2中:
SQL> set autotrace on
SQL> select * from test;
ID
----------
1000
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
----------------------------------------------------------
0 recursive calls
0 db block gets
9 consistent gets 没有事物做update时 是 7 consistent gets 说明多了2个 consistent gets 这2个是要从回滚段中获取的
0 physical reads
52 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
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>
