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Todd — Tue, 24 Jul 2012 18:29:00 GMT

�U�d��一个业务高�q�发�Q�接入一个客��L��话就发一个特�D�邀��L��Q�码为字�W�加数字��L��Q�无规律�Q�，码已存表�Q�业务上�?码和用户手机��L��一一对应�Q?br /> 所以，来个用户��得�Ҏ��标记的码更新手机��L��Q�高�q�发�Ӟ��行锁及等待比较耗时�Q�导致数据库性能下降严重�Q?br /> 解决办法�Q?br /> 码表增加seqid,字段�Q�导入数据时使用rownum,做该字段�?建烦引；
更新�Ӟ��使用序列�Q?

CREATE SEQUENCE SQ_U_SEQ INCREMENT BY 1 START WITH 1 MAXVALUE 50000000 CYCLE CACHE 2000 NOORDER;
更新则：update ... where ... and t.seqid = SQ_U_SEQ.nextval ...;
以后有新的数据要导入�Q�则序列需重置�Q�则�Q?/div>

create or replace procedure seq_reset(v_seqname varchar2) as n number(10);
tsql varchar2(100);
begin
execute immediate 'select '||v_seqname||'.nextval from dual' into n;
  n:=-(n-1);
  tsql:='alter sequence '||v_seqname||' increment by '|| n;
  execute immediate tsql;
execute immediate 'select '||v_seqname||'.nextval from dual' into n;
  tsql:='alter sequence '||v_seqname||' increment by 1';
execute immediate tsql;
end seq_reset;

另外�Q�高�q�发时序列的cache要设�|�大�?一般业务设�|?00以上�Q�但nocache的时候性能��实很差�Q�最大相�?0倍．排序参数�Q�oracle默认是NOORDER�Q�如果设�|��ؓORDER�Q�在单实例环境没有媄响，在RAC环境此时�Q�多实例实际�~�存相同的序列，此时在多个实例�ƈ发取该序列的时候，会有短暂的资源竞争来在多实例之间�q�行同步。因�ơ性能相比noorder要差�Q�所以RAC环境非必��ȝ��情况下不要��用ORDER�Q�尤其要避免NOCACHE ORDER�l�合�Q?

Todd 2012-07-25 02:29 发表评论

Todd — Tue, 20 Sep 2011 18:49:00 GMT

一。job的运行频率设�|?/font>

1.每天固定旉��q�行�Q�比如早�?:10分钟�Q�Trunc(Sysdate+1) + (8*60+10)/24*60

2.Toad中提供的�Q?/p>

每天�Q�trunc(sysdate)+1

每周�Q�trunc(sysdate)+7

每月�Q�trunc(sysdate)+30

每个星期日：next_day(trunc(sysdate),'SUNDAY')

每天6点：trunc(sysdate)+1+6/24

半个��时�Q�sysdate+30/1440

3.每个��时的第15分钟�q�行�Q�比如：8:15�Q?:15�Q?0:15...�Q�trunc(sysdate,'hh')+75/1440。原先我讄��的是trunc(sysdate,'hh')+15/1440�Q�发现居然不行�?/p>

二。JOB��Z��么不�q�行�Q?/font>

1.先来了解一下JOB的参数说明：与job相关的参��C��个是job_queue_processes�Q�这个是�q�行JOB时候所��L��q�程敎ͼ�当然�pȝ��里面JOB大于�q�个数值后�Q�就会有排队�{�候的�Q�最��值是0�Q�表�C�Z��q�行JOB�Q�最大值是36�Q�在OS上对应的�q�程时SNPn�Q?i以后OS上管理JOB的进�E�叫CJQn。可以��用下面这个SQL��定目前有几个SNP/CJQ在运行�?/p>

select * from v$bgprocess�Q�这个paddr不�ؓ�I�的snp/cjq�q�程��是目前�I�闲的进�E�，有的表示正在工作的进�E��?/p>

另外一个是job_queue_interval�Q�范围在1--3600之间�Q�单位是�U�，�q�个是唤醒JOB的process�Q�因为每�ơsnp�q�行完他��׃��息了�Q�需要定期唤醒他�Q�这个��g��能太��，太小会媄响数据库的性能�?/p>

2.诊断�Q�先��定上面�q�两个参数设�|�是否正��，特别是第一个参敎ͼ�讄��?了，所有JOB��׃��会跑�Q�确认无误后�Q�我们��l�向下�?/p>

3.使用下面的SQL察看JOB的的broken,last_date和next_date�Q�last_date是指最�q�一�ơjob�q�行成功的结束时��_��next_date是根据设�|�的频率计算的下�ơ执行时��_��Ҏ��q�个信息��可以判断JOB上次是否正常�Q�还可以判断下次的时间对不对�Q�SQL如下�Q?/p>

select * from dba_jobs

有时候我们发��C��的next_date�?000�q?�?日，说明job要不��是在running�Q�要不就是状态是break(broken=Y)�Q�如果发现JOB的broken��gؓY�Q�找用户了解一下，��定该JOB是否可以broken�Q�如果不能broken�Q�那��把broken��g��Ҏ��N�Q�修改再使用上面的SQL察看��发��C��的last_date已经变了�Q�JOB卛_��正常�q�行�Q�修改broken状态的SQL如下�Q?/p>

declare

BEGIN

DBMS_JOB.BROKEN(,FALSE);

END;

4.使用下面的SQL查询是否JOB�q�在Running

select * from dba_jobs_running

如果发现JOB已经Run了很久了�q�没有结束，��p��查原因了。一般的JOB running时会锁定相关的相关的资源�Q�可以查看一下v$access和v$locked_object�q�两个view�Q�如果发现其他进�E�锁定了与JOB相关的Object�Q�包括PKG/Function/Procedure/Table�{�资源，那么��p��把其他进�E�删除，有必要的话，把JOB的进�E�也删除�Q�再重新跑看看结果�?/p>

5.如果上面都正常，但是JOB�q�不run�Q�怎么办？那我们要考虑把JOB�q�程重启一�ơ，防止是SNP�q�程��M��造成JOB不跑�Q�指令如下：

alter system set job_queue_processes=0 --关闭job�q�程�Q�等�?--10�U�钟

alter system set job_quene_processes=5 --恢复原来的�?/p>

6.Oracle的BUG

Oracle9i里面有一个BUG�Q�当计数器到497天时�Q�刚好达到它的最大��|��再计数就会变�?1�Q��l�计数就变成0了，然后计数器将不再跑了。如果碰到这�U�情况就得重启数据库�Q�我们这�Ҏ��一个生产型的数据库版本�?205�Q�就发生�q�这样一�ơ问题，后来和用��L��旉��重启后就没问题了。但是其他的Oracle7345和Oracle8i的数据库没有发现�q�个问题�?/p>

7.数据库上的检查基本上��p��多，如果JOB�q�行�q�有问题�Q�那需要配合用户察看一下是否是�E�序本��n的问题，比如处理的资料量大，或者网�l�速度慢等造成�q�行时过长，那就需要具体情况具体分析了。我们可以通过下面的SQL手工执行一下JOB看看�Q?/p>

declare

begin

dbms_job.run(_ID)

end;

如果发现JOB执行不正常，��p��l�合�E�序具体分析一下�?/p>

Todd 2011-09-21 02:49 发表评论

[转]linux 启动 oracle

Todd — Mon, 18 Apr 2011 08:05:00 GMT

启动数据库实例，分�ؓ两步�Q�第一步，启动监听�Q�第二步�Q�启动数据库实例�?br />
一、如何启动数据库实例
   1.�q�入到sqlplus启动实例
[oracle@redhat ~]$ su - oracle                                 --“切换到oracle用户”
Password:
[oracle@redhat ~]$ lsnrctl start                               --“打开监听”
LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 14-OCT-2009 19:06:40
Copyright (c) 1991, 2005, Oracle. All rights reserved.
Starting /home/oracle/product/10g/bin/tnslsnr: please wait...
TNSLSNR for Linux: Version 10.2.0.1.0 - Production
System parameter file is /home/oracle/product/10g/network/admin/listener.ora
Log messages written to /home/oracle/product/10g/network/log/listener.log
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC2)))
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=redhat)(PORT=1522)))
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC2)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 10.2.0.1.0 - Production
Start Date                14-OCT-2009 19:06:40
Uptime                    0 days 0 hr. 0 min. 0 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Parameter File   /home/oracle/product/10g/network/admin/listener.ora
Listener Log File         /home/oracle/product/10g/network/log/listener.log
Listening Endpoints Summary...
(DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC2)))
(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=redhat)(PORT=1522)))
Services Summary...
Service "PLSExtProc" has 1 instance(s).
Instance "PLSExtProc", status UNKNOWN, has 1 handler(s) for this service...
The command completed successfully
[oracle@redhat ~]$ sqlplus /nolog                                --“�q�入到sqlplus”
SQL*Plus: Release 10.2.0.1.0 - Production on Wed Oct 14 19:06:45 2009
Copyright (c) 1982, 2005, Oracle. All rights reserved.
SQL> conn /as sysdba                                              --“�q�接到sysdba”
Connected to an idle instance.
SQL> startup                                                     --“启动数据库实�?#8221;
ORACLE instance started.
Total System Global Area 285212672 bytes
Fixed Size      1218968 bytes
Variable Size     88082024 bytes
Database Buffers   188743680 bytes
Redo Buffers      7168000 bytes
Database mounted.
Database opened.
SQL> shutdown immediate                                          --“关闭数据库实�?#8221;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
[oracle@redhat ~]$ lsnrctl stop                                 --“关闭监听”
LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 14-OCT-2009 19:08:06
Copyright (c) 1991, 2005, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC2)))
The command completed successfully

   2.用dbstart和dbshut启动和关闭数据库实例

先启动监�?lsnrctl start
启动实例 dbstart

使用dbstart命��o启动数据库比较方便，但是在linux上安装好oracle之后�Q�第一�ơ��用dbstart命��o可能会报如下错误�Q?br />
ORACLE_HOME_LISTNER is not SET, unable to auto-start Oracle Net Listener
Usage: /u01/app/oracle/oracle/product/10.2.0/db_1/bin/dbstart ORACLE_HOME

原因�Q?br /> dbstart和dbshut脚本文�g中ORACLE_HOME_LISTNER的设�|�有问题�Q�分别打开两个文�g扑ֈ��Q�用vi�~�辑dbstart�Q�ORACLE_HOME_LISTNER=$1,修改�?br /> ORACLE_HOME_LISTNER=$ORACLE_HOME

然后保存退出，此时再运行dbstart�Q�已�l�不报错了，但是没有��M��反应�Q�ps一下进�E�，没有oracle的进�E�，说明oracle实例没有正常启动�?br />
此时的原因是�?etc/oratab的设�|�问题，我们vi一下，发现
zgz:/home/oracle/product/10g:N
最后设�|�的�?N"�Q�我的环境中只有一个实例，因此只有一行配�|�语句）�Q�我们需要把“N”修改�?#8220;Y”�?br />
以上的工作做好之后，dbstart��可以正�怋�用了�Q?br />
[oracle@redhat bin]$ lsnrctl start                                   --“启动监听”
LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 14-OCT-2009 19:44:53
Copyright (c) 1991, 2005, Oracle. All rights reserved.
Starting /home/oracle/product/10g/bin/tnslsnr: please wait...
TNSLSNR for Linux: Version 10.2.0.1.0 - Production
System parameter file is /home/oracle/product/10g/network/admin/listener.ora
Log messages written to /home/oracle/product/10g/network/log/listener.log
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC2)))
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=redhat)(PORT=1522)))
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC2)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 10.2.0.1.0 - Production
Start Date                14-OCT-2009 19:44:53
Uptime                    0 days 0 hr. 0 min. 0 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Parameter File   /home/oracle/product/10g/network/admin/listener.ora
Listener Log File         /home/oracle/product/10g/network/log/listener.log
Listening Endpoints Summary...
(DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC2)))
(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=redhat)(PORT=1522)))
Services Summary...
Service "PLSExtProc" has 1 instance(s).
Instance "PLSExtProc", status UNKNOWN, has 1 handler(s) for this service...
The command completed successfully
[oracle@redhat bin]$ dbstart                                   --“启动数据库实�?#8221;
Processing Database instance "zgz": log file /home/oracle/product/10g/startup.log
[oracle@redhat bin]$ dbshut                                    --“关闭数据库实�?#8221;
[oracle@redhat bin]$ lsnrctl stop                              --“关闭监听”
LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 14-OCT-2009 19:45:33
Copyright (c) 1991, 2005, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC2)))
The command completed successfully

二、如何��数据库实例和linux�pȝ��一起启�?br /> �?etc/rc.d/rc.local中加入如下语句即可实现同�pȝ��启动实例�Q?br /> su - oracle -c "lsnrctl start"
su - oracle -c "dbstart"

Todd 2011-04-18 16:05 发表评论

Todd — Fri, 15 Apr 2011 08:15:00 GMT

隔离�U�别�Q�isolation level�Q?/span>

l 隔离�U�别定义了事务与事务之间的隔��ȝ��度�?/span>

l 隔离�U�别与�ƈ发性是互�ؓ矛盾的：隔离�E�度��高�Q�数据库的�ƈ发性越差；隔离�E�度��低�Q�数据库的�ƈ发性越好�?/span>

l ANSI/ISO SQL92标准定义了一些数据库操作的隔��ȝ�别：

l 未提交读�Q�read uncommitted�Q?/span>

l 提交读（read committed�Q?/span>

l 重复读（repeatable read�Q?/span>

l 序列化（serializable�Q?/span>

l 通过一些现象，可以反映出隔��ȝ�别的效果。这些现象有�Q?/span>

l 更新丢失�Q�lost update�Q�：当系�l�允�怸�个事务同时更新同一数据是，发生更新丢失�?/span>

l 脏读�Q�dirty read�Q�：当一个事务读取另一个事务尚未提交的修改�Ӟ��产生脏读�?/span>

l 非重复读�Q�nonrepeatable read�Q�：同一查询在同一事务中多�ơ进行，�׃��其他提交事务所做的修改或删除，每次�q�回不同的结果集�Q�此时发生非重复诅R�?A transaction rereads data it has previously read and finds that another committed transaction has modified or deleted the data. )

l �q�d��Q�phantom read�Q�：同一查询在同一事务中多�ơ进行，�׃��其他提交事务所做的插入操作�Q�每�ơ返回不同的�l�果集，此时发生�q�d��诅R�?A transaction reexecutes a query returning a set of rows that satisfies a search condition and finds that another committed transaction has inserted additional rows that satisfy the condition. )

l 下面是隔��ȝ�别及其对应的可能出现或不可能出现的现�?/span>

	Dirty Read	NonRepeatable Read	Phantom Read
Read uncommitted	Possible	Possible	Possible
Read committed	Not possible	Possible	Possible
Repeatable read	Not possible	Not possible	Possible
Serializable	Not possible	Not possible	Not possible

ORACLE的隔��ȝ��?/span>

l ORACLE提供了SQL92标准中的read committed和serializable�Q�同时提供了非SQL92标准的read-only�?/span>

l read committed�Q?/span>

l �q�是ORACLE�~�省的事务隔��ȝ�别�?/span>

l 事务中的每一条语句都遵从语句�U�的��M��致性�?/span>

l 保证不会脏读�Q�但可能出现非重复读和��像�?/span>

l serializable�Q?/span>

l ��单地��_��serializable��是使事务看��h��象是一个接着一个地��序地执行�?/span>

l 仅仅能看见在本事务开始前由其它事务提交的更改和在本事务中所做的更改�?/span>

l 保证不会出现非重复读和��像�?/span>

l Serializable隔离�U�别提供了read-only事务所提供的读一致性（事务�U�的��M��致性）�Q�同时又允许DML操作�?/span>

l 如果有在serializable事务开始时未提交的事务在serializable事务�l�束之前修改了serializable事务��要修改的行�q�进行了提交�Q�则serializable事务不会��d��q�些变更�Q�因此发生无法序列化讉K��的错误。（换一�U�解释方法：只要在serializable事务开始到�l�束之间有其他事务对serializable事务要修改的东西�q�行了修改�ƈ提交了修改，则发生无法序列化讉K��的错误。）

l If a serializable transaction contains data manipulation language (DML) that attempts to update any resource that may have been updated in a transaction uncommitted at the start of the serializable transaction, �Q��ƈ且修改在后来被提交而没有回滚）�Q�then the DML statement fails. �q�回的错误是ORA-08177: Cannot serialize access for this transaction�?/span>

l ORACLE在数据块中记�?u>最�q�对数据行执行修�Ҏ��作的N个事�?/u>的信息，目的是确定是否有在本事务开始时未提交的事务修改了本事务��要修改的行。具体见英文�Q�Oracle permits a serializable transaction to modify a data row only if it can determine that prior changes to the row were made by transactions that had committed when the serializable transaction began. To make this determination efficiently, Oracle uses control information stored in the data block that indicates which rows in the block contain committed and uncommitted changes. In a sense, the block contains a recent history of transactions that affected each row in the block. The amount of history that is retained is controlled by the INITRANS parameter of CREATE TABLE and ALTER TABLE. Under some circumstances, Oracle may have insufficient history information to determine whether a row has been updated by a "too recent" transaction. This can occur when many transactions concurrently modify the same data block, or do so in a very short period. You can avoid this situation by setting higher values of INITRANS for tables that will experience many transactions updating the same blocks. Doing so will enable Oracle to allocate sufficient storage in each block to record the history of recent transactions that accessed the block.

l The INITRANS Parameter�Q�Oracle stores control information in each data block to manage access by concurrent transactions. Therefore, if you set the transaction isolation level to serializable, you must use the ALTER TABLE command to set INITRANS to at least 3. This parameter will cause Oracle to allocate sufficient storage in each block to record the history of recent transactions that accessed the block. Higher values should be used for tables that will undergo many transactions updating the same blocks.

l read-only�Q?/span>

l 遵从事务�U�的��M��致性，仅仅能看见在本事务开始前由其它事务提交的更改�?/span>

l 不允许在本事务中�q�行DML操作�?/span>

l read only是serializable的子集。它们都避免了非重复��d��q�d��。区别是在read only中是只读�Q�而在serializable中可以进行DML操作�?/span>

l Export with CONSISTENT = Y sets the transaction to read-only.

l read committed和serializable的区别和联系�Q?/span>

l 事务1先于事务2开始，�q�保持未提交状态。事�?惌��修改正被事务1修改的行。事�?�{�待。如果事�?回滚�Q�则事务2�Q�不论是read committed�q�是serializable方式�Q�进行它惌��做的修改。如果事�?提交�Q�则当事�?是read committed方式�Ӟ��q�行它想要做的修改；当事�?是serializable方式�Ӟ��p�|�q�报�?#8220;Cannot serialize access”�Q?span style="color: red">因�ؓ事务2看不见事�?提交的修改，且事�?惛_��事务一修改的基��上再做修�?/span>。具体见英文�Q�Both read committed and serializable transactions use row-level locking, and both will wait if they try to change a row updated by an uncommitted concurrent transaction. The second transaction that tries to update a given row waits for the other transaction to commit or roll back and release its lock. If that other transaction rolls back, the waiting transaction (regardless of its isolation mode) can proceed to change the previously locked row, as if the other transaction had not existed. However, if the other (blocking) transaction commits and releases its locks, a read committed transaction proceeds with its intended update. A serializable transaction, however, fails with the error "Cannot serialize access", because the other transaction has committed a change that was made since the serializable transaction began.

l read committed和serializable可以在ORACLE�q�行服务器中使用�?/span>

l 关于SET TRANSACTION READ WRITE�Q�read write和read committed 应该是一��L��。在��L��面，它们都避免了脏读�Q�但都无法实现重复读。虽然没有文档说明read write在写斚w��与read committed一��_��但显然它在写的时候会加排他锁以避免更��C��失。在加锁的过�E�中�Q�如果遇到待锁定资源无法锁定�Q�应该是�{�待而不是放弃。这与read committed一致�?/span>

l 语句�U�的��M��致�?/span>

l ORACLE保证语句�U�的��M��致性，即一个语句所处理的数据集是在单一旉��点上的数据集�Q�这个时间点是这个语句开始的旉��?/span>

l 一个语句看不见在它开始执行后提交的修攏V�?/span>

l 对于DML语句�Q�它看不见由自己所做的修改�Q�即DML语句看见的是它本�w�开始执行以前存在的数据�?/span>

l 事务�U�的��M��致�?/span>

l 事务�U�的��M��致性保证了可重复读�Q��ƈ保证不会出现�q�d��?/span>

l 讄��隔离�U�别

l 讄��一个事务的隔离�U�别

l SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

l SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

l SET TRANSACTION READ ONLY;

l 讄��增个会话的隔��ȝ��?/span>

l ALTER SESSION SET ISOLATION_LEVEL SERIALIZABLE;

l ALTER SESSION SET ISOLATION_LEVEL READ COMMITTED;

Todd 2011-04-15 16:15 发表评论

mysql事务隔离�U�别以及有问题的��d��Q�脏读，不可重复读，�q�象读）

Todd — Sat, 11 Sep 2010 07:50:00 GMT

1.事务里一些有问题的读取：脏读�Q�不可重复读�Q��象读

脏读 �Q�dirty read�Q�事务T1更新了一行记录的内容�Q�但是�ƈ没有提交所做的修改。事务T2��d��更新后的行，然后T1执行回滚操作�Q�取消了刚才所做的修改。现在T2所��d��的行��无效了�?/p>

不可重复��d�� Q�nonrepeatable read�Q�事务T1��d��一行记录，紧接着事务T2修改了T1刚才��d��的那一行记录。然后T1又再�ơ读取这行记录，发现与刚才读取的�l�果不同。这��q��为“不可重复”读�Q�因为T1原来��d��的那行记录已�l�发生了变化�?/p>

�q�d��d�� Q�phantom read�Q�事务T1��d��一条指定的WHERE子句所�q�回的结果集。然后事务T2新插�?一行记录，�q�行记录恰好可以满��T1所使用的查询条件中的WHERE 子句的条件。然后T1又��用相同的查询再次对表�q�行��索，但是此时却看��C��事务T2刚才插入的新行。这个新行就�U�Cؓ“��像”，因�ؓ对T1来说�q�一行就像突然出现的一栗��?/p>

2.事务的隔��ȝ��?/p>

从��别低到高依次为：

READ UNCOMMITTED �q�d��诅R��不可重复读和脏读都允许�?/p>

READ COMMITTED 允许�q�d��诅R��不可重复读�Q�但不允许脏诅R�?/p>

REPEATABLE READ 允许�q�d��读，但不允许不可重复��d��脏读。InnoDB默认�U�别

SERIALIZABLE �q�d��诅R��不可重复读和脏读都不允许�?/p>

但是InnoDB的可重复读隔��ȝ�别和其他数据库的可重复读是有区别的，不会造成�q�象读（phantom read�Q��?/p>

ORACLE数据库支�?READ COMMITTED �?SERIALIZABLE �Q�不支持 READ UNCOMMITTED �?REPEATABLE READ �?/p>

3.��试�Q?/p>

1�Q��ƈ发更�?�Q�表tab1 的一条记录id=1 num=1�Q�两个session分别执行事务1�Q?

分别输入下面的语�?/p>

事务1 (session1) �Q?/p>

start transaction;

update tab1 set num=num+1 where id=1�Q?/p>

=========

一条记录被更新�Q�select发现num=2;

事务2 (session2)�Q?/p>

start transaction;

update tab1 set num=num+1 where id=1;

=========

事务2会被��d��Q�然后session1输入commit;提交事务1。此时事�?更新成功�?/p>

session1 select一下会发现num�q�是2�Q�不允许脏读 �Q�；session2 select 一�?num �?�Q�update可以得到最新提交过的数据然后更斎ͼ�但是如果没有update�Q�一直是select 的话select得到的num一直是1 �Q�，然后commit�Q?/p>

session1 select 发现num�? �Q�session2 select 发现num�?�Q?/p>

所以，事务中update是有行写锁（排他锁）的，不会发生�Q?脏读和不可重复读�Q�对于自�w�有update的事务，update之后是可以读到最新数据的�Q�这属于例外�Q�^_^ �Q�，

2�Q�关于��象读 �Q�innodb默认事务隔离�U�别是不会出现的�Q�摘自网上，��试通过�Q?/p>

mysql> set autocommit=off;
Query OK, 0 rows affected (0.00 sec)

session 1 创徏表�ƈ插入��试数据

mysql> create table test(i int) engine=innodb;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into test values(1);
Query OK, 1 row affected (0.00 sec)
session 2 查询�Q�没有数据，正常�Q�session1没有提交�Q�不允许脏读

mysql> select * from test;
Empty set (0.00 sec)
session 1 提交事务

mysql> commit;
Query OK, 0 rows affected (0.00 sec)
session 2 查询�Q�还是没有数据，没有产生�q�象�?/p>

mysql> select * from test;
Empty set (0.00 sec)
4.最后，现在应该知道数据库�ƈ发控制�ƈ不复杂，交给数据库的事务��p��Q�mysql 选用innodb引擎�Q�不会出玎ͼ�“不可重复读”（��是在事�?�q�行数据修改的时候，事务2��d��的数据是没修�?之前的数据，事务1提交的时候，事务2再次��d��的时候得到的是修改后的数据，单个事务内所��L��据不一��_��Q�“脏诠Z��，“��象读”；

关于innodb锁机制的详细解释�Q�参见《mysql手册�?�?5.2.10 . InnoDB 事务模型和锁定”�?

本文来自CSDN博客�Q��{载请标明出处�Q?a >http://blog.csdn.net/todd_liu/archive/2010/09/10/5875650.aspx

Todd 2010-09-11 15:50 发表评论

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mysql事务隔离�U�别以及有问题的��d���Q�脏读，不可重复读，�q�象读）

mysql事务隔离�U�别以及有问题的��d��Q�脏读，不可重复读，�q�象读）