1. 程式人生 > >oracle--select for update nowait 與 select for update 區別

oracle--select for update nowait 與 select for update 區別

nowait的含義很多人都會誤解為“不用等待,立即執行”。但實際上該關鍵字的含義是“不用等待,立即返回”
如果當前請求的資源被其他會話鎖定時,會發生阻塞,nowait可以避免這一阻塞,因為
If another user is in the process of modifying that row, we will get an ORA‐00054
Resource Busy error. We are blocked and must wait for the other user to finish with
it.
可以實驗下,我用pl/sql developer鎖定表game
SQL> select * from game where game_id =1;
返回一條記錄

SQL> select * from game where game_id=1 for update nowait;
select * from game where game_id=1 for update nowait
*
ERROR位於第1行:
ORA-00054:資源正忙,要求指定NOWAIT
使用NOWAIT關鍵字,會報ORA‐00054的錯誤


如何來檢視是什麼資源造成這樣的情況呢?並且怎麼解決呢?
檢視鎖定的物件,使用者和會話
SQL> select lo.oracle_username,do.object_name,s.logon_time,lo.process,s.sid as s
ession_id
2 from v$locked_object lo,v$session s,dba_objects do
3 where lo.session_id = s.sid and do.object_id = lo.OBJECT_ID
4 /
ORACLE_USERNAME
------------------------------
OBJECT_NAME
--------------------------------------------------------------------------------
LOGON_TIME PROCESS SESSION_ID
------------------- ------------ ----------
NBA---使用者名稱稱
GAME---操作的物件
2009-08-04 10:55:15---登入的時間 840:5176 10


使用as sysdba
根據sid檢視具體的sql語句
selectsql_textfromv$session a,v$sqltext_with_newlines b
whereDECODE(a.sql_hash_value,0, prev_hash_value, sql_hash_value)=b.hash_value
anda.sid=10;
begin :id := sys.dbms_transaction.local_transaction_id; end;

kill session
SQL> select sid,serial# from v$session where sid =10;

SID SERIAL#
---------- ----------
10 23
SQL> alter system kill session '10,23';

系統已更改。

select * from game where game_id=1 for update nowait;
有資料返回了

當兩個使用者同時更新同一條記錄是, 使用select for update,後執行者,會被阻塞,而使用select for update nowait 則會丟擲:ORA-00054 resource busy and acquire with NOWAIT specified 異常,告之使用者這一行已經鎖定。

原文來源:[url]http://www.blogjava.net/parable-myth/archive/2010/11/05/337350.html?opt=admin[/url]