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1.物理讀(physical read)

物理讀即是把數(shù)據(jù)從磁盤讀入到buffer catch的過程。 通常情況下是，如果需要數(shù)據(jù)的時候發(fā)現(xiàn)不存在于buffer catch當中，即oracle就會執(zhí)行物理讀。 

當數(shù)據(jù)塊***次讀取到,就會緩存到buffer cache 中,而第二次讀取和修改該數(shù)據(jù)塊時就在內(nèi)存buffer cache 了 以下是例子:

1.1 ***次讀取:

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個物理讀

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 第二次讀取

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 --沒有發(fā)生物理讀了,直接從buffer cache 中讀取了

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 數(shù)據(jù)塊被重新讀入buffer cache ,這種發(fā)生在

如果有新的數(shù)據(jù)需要被讀入Buffer Cache中，而Buffer Cache又沒有足夠的空閑空間，Oracle就根據(jù)LRU算法將LRU鏈表中LRU端的數(shù)據(jù)置換出去。當這些數(shù)據(jù)被再次訪問到時，需要重新從磁盤讀入。

SQL> alter session set events 'immediate trace name flush_cache'; --清空數(shù)據(jù)緩沖區(qū)

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 --又重新發(fā)生了物理讀

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.邏輯讀(buffer read)

邏輯讀指的就是從(或者視圖從)Buffer Cache中讀取數(shù)據(jù)塊。按照訪問數(shù)據(jù)塊的模式不同，可以分為即時讀(Current Read)和一致性讀(Consistent Read)。注意：邏輯IO只有邏輯讀，沒有邏輯寫。

即時讀

即時讀即讀取數(shù)據(jù)塊當前的***數(shù)據(jù)。任何時候在Buffer Cache中都只有一份當前數(shù)據(jù)塊。即時讀通常發(fā)生在對數(shù)據(jù)進行修改、刪除操作時。這時，進程會給數(shù)據(jù)加上行級鎖，并且標識數(shù)據(jù)為“臟”數(shù)據(jù)。

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>

一致性讀

Oracle是一個多用戶系統(tǒng)。當一個會話開始讀取數(shù)據(jù)還未結束讀取之前，可能會有其他會話修改它將要讀取的數(shù)據(jù)。如果會話讀取到修改后的數(shù)據(jù)，就會造成數(shù)據(jù)的不一致。一致性讀就是為了保證數(shù)據(jù)的一致性。在Buffer Cache中的數(shù)據(jù)塊上都會有***一次修改數(shù)據(jù)塊時的SCN。如果一個事務需要修改數(shù)據(jù)塊中數(shù)據(jù)，會先在回滾段中保存一份修改前數(shù)據(jù)和SCN的數(shù)據(jù)塊，然后再更新Buffer Cache中的數(shù)據(jù)塊的數(shù)據(jù)及其SCN，并標識其為“臟”數(shù)據(jù)。當其他進程讀取數(shù)據(jù)塊時，會先比較數(shù)據(jù)塊上的SCN和自己的SCN。如果數(shù)據(jù)塊上的SCN小于等于進程本身的SCN，則直接讀取數(shù)據(jù)塊上的數(shù)據(jù);如果數(shù)據(jù)塊上的SCN大于進程本身的SCN，則會從回滾段中找出修改前的數(shù)據(jù)塊讀取數(shù)據(jù)。通常，普通查詢都是一致性讀。

下面這個例子幫助大家理解一下一致性讀：

會話1中：

SQL> select * from test;

ID

----------

1000

SQL> update test set id=2000;

1 row updated.

會話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 沒有事物做update時 是 7 consistent gets 說明多了2個 consistent gets 這2個是要從回滾段中獲取的

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>