Oracle数据库中加锁应该都是悲观锁是吗？

╰+攻爆jí腚メ 2022-05-28 12:59 186阅读 0赞

# Oracle数据库中加锁应该都是悲观锁是吗？ \[问题点数：40分，结帖人chenchendf\] #

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     </div> 我的理解是在数据库中加的锁都是悲观锁，如select&nbsp;..&nbsp;for&nbsp;update,&nbsp;insert等。 
     <br style=""> 
     <br style=""> 数据库中可以加乐观锁吗？如有，能否举个例子？ 
     <br style=""> 
     <br style=""> 基本的锁类型有两种：排它锁（Exclusive&nbsp;locks记为X锁）和共享锁（Share&nbsp;locks记为S锁）。排它锁应该是悲观锁，共享锁是属于悲观还是乐观锁？ 
     <br style=""> 
     <br style=""> 交叉概念有点晕，以前知道的悲观锁和乐观锁是在hibernate中的，但在百度搜索的时候看到很多oracle的悲观锁与乐观锁。&nbsp; 
     <br style=""> 
     <br style=""> 请求高手，莫见笑啊~~ 
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     <span style="">发表于： 2011-09-01 15:31:59&nbsp;</span> 
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      Oracle中，我不知道悲观锁和乐观锁，不经常这么叫的吧？ 
     <br style=""> 只知道你说的排它锁之类的。 
     <br style=""> 
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     <span style="">回复于： 2011-09-01 16:22:59</span> 
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    <div style="list-style:none; margin:0px; padding:16px; border:none; outline:none 0px; word-wrap:break-word; overflow:hidden; font-size:14px; line-height:28px; width:758px">   引用&nbsp;4&nbsp;楼&nbsp;chenchendf&nbsp;的回复: 
     <blockquote style="padding:0px; margin:0px; font-size:14px; border:none; list-style:none; outline:none 0px">
       引用&nbsp;2&nbsp;楼&nbsp;wuhuipengwhp&nbsp;的回复: 
      <br style=""> 有悲观锁也有乐观锁的 
      <br style=""> 
      <br style=""> 能否举个乐观锁的例子？在oracle数据库中的。 
     </blockquote> SQL&gt;&nbsp;desc&nbsp;t 
     <br style=""> &nbsp;名称&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;是否为空?&nbsp;类型 
     <br style=""> &nbsp;-----------------------------------------&nbsp;--------&nbsp;------------------ 
     <br style=""> &nbsp;I&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;NUMBER(38) 
     <br style=""> 
     <br style=""> SQL&gt;&nbsp;select&nbsp;*&nbsp;from&nbsp;t&nbsp;where&nbsp;rownum=1; 
     <br style=""> 
     <br style=""> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;I 
     <br style=""> ---------- 
     <br style=""> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;258 
     <br style=""> 
     <br style=""> SQL&gt;&nbsp;update&nbsp;t&nbsp;set&nbsp;i=255&nbsp;where&nbsp;i=258; 
     <br style=""> 
     <br style=""> 已更新&nbsp;1&nbsp;行。 
     <br style=""> 
     <br style=""> update语句里面加where条件，tom书里面说的大概就是这个意思。 
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      <li style="list-style:none; margin:0px; padding:0px; border:none; outline:none 0px; width:820px; line-height:25px; overflow:hidden; text-overflow:ellipsis; white-space:nowrap"> <a href="https://blog.csdn.net/sadfishsc/article/details/51027734" title="MySQL InnoDB 悲观锁机制" style="background:transparent; color:rgb(51,90,164)" rel="nofollow">MySQL InnoDB 悲观锁机制</a></li> 
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     <span style="">回复于： 2011-09-01 21:27:06</span> 
     <span style="float:right; margin-left:50px"><a href="https://bbs.csdn.net/topics/370165246#post-380587183" style="background:transparent; color:rgb(102,102,102); margin:0px 2px" rel="nofollow">#6</a>&nbsp;得分：5</span> 
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    <div style="list-style:none; margin:0px; padding:16px; border:none; outline:none 0px; word-wrap:break-word; overflow:hidden; font-size:14px; line-height:28px; width:758px">   引用&nbsp;6&nbsp;楼&nbsp;wffffc&nbsp;的回复: 
     <blockquote style="padding:0px; margin:0px; font-size:14px; border:none; list-style:none; outline:none 0px">
       SQL&gt;&nbsp;desc&nbsp;t 
      <br style=""> &nbsp;名称&nbsp;是否为空?&nbsp;类型 
      <br style=""> &nbsp;-----------------------------------------&nbsp;--------&nbsp;------------------ 
      <br style=""> &nbsp;I&nbsp;NUMBER(38) 
      <br style=""> 
      <br style=""> SQL&gt;&nbsp;select&nbsp;*&nbsp;from&nbsp;t&nbsp;where&nbsp;rownum=1; 
      <br style=""> 
      <br style=""> &nbsp;&nbsp;I 
      <br style=""> ---------- 
      <br style=""> &nbsp;&nbsp;258 
      <br style=""> 
      <br style=""> SQL&gt;&nbsp;update&nbsp;t&nbsp;set&nbsp;i=255&nbsp;where&nbsp;i=258; 
      <br style=""> 
      <br style=""> 已更新&nbsp;1&nbsp;行。 
      <br style=""> 
      <br style=""> update语句里面加where条件，tom书里面说的大概就是这个意思。 
     </blockquote> 没看出哪里加了乐观锁啊？&nbsp;update语句加where条件就是乐观锁？ 
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      <li style="list-style:none; margin:0px; padding:0px; border:none; outline:none 0px; width:820px; line-height:25px; overflow:hidden; text-overflow:ellipsis; white-space:nowrap"> <a href="https://blog.csdn.net/heronos/article/details/77896050" title="数据库中悲观锁和乐观锁" style="background:transparent; color:rgb(51,90,164)" rel="nofollow">数据库中悲观锁和乐观锁</a></li> 
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     <span style="">回复于： 2011-09-02 08:57:20</span> 
     <span style="float:right; margin-left:50px"><a href="https://bbs.csdn.net/topics/370165246#post-380590626" style="background:transparent; color:rgb(102,102,102); margin:0px 2px" rel="nofollow">#7</a>&nbsp;得分：0</span> 
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    <div style="list-style:none; margin:0px; padding:16px; border:none; outline:none 0px; word-wrap:break-word; overflow:hidden; font-size:14px; line-height:28px; width:758px">   引用楼主&nbsp;chenchendf&nbsp;的回复: 
     <blockquote style="padding:0px; margin:0px; font-size:14px; border:none; list-style:none; outline:none 0px">
       我的理解是在数据库中加的锁都是悲观锁，如select&nbsp;..&nbsp;for&nbsp;update,&nbsp;insert等。 
      <br style=""> 
      <br style=""> 数据库中可以加乐观锁吗？如有，能否举个例子？ 
      <br style=""> 
      <br style=""> 基本的锁类型有两种：排它锁（Exclusive&nbsp;locks记为X锁）和共享锁（Share&nbsp;locks记为S锁）。排它锁应该是悲观锁，共享锁是属于悲观还是乐观锁？ 
      <br style=""> 
      <br style=""> 交叉概念有点晕，以前知道的悲观锁和乐观锁是在hibernate中的，但在…… 
     </blockquote> &nbsp;oracle有悲观锁也有乐观锁。&nbsp;悲观锁比较安全一些，可以防止丢失更新，但是就是互相等待，影响效率。 
     <br style=""> 一般会用乐观锁，即开始操作时，乐观的认为数据不会被其他人更改，直到提交时才加锁检查。比如在操作的表上加一列，保存个时间戳，提交时检查是不是最新的。不过乐观锁失败的可能性比较大。 
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      <li style="list-style:none; margin:0px; padding:0px; border:none; outline:none 0px; width:820px; line-height:25px; overflow:hidden; text-overflow:ellipsis; white-space:nowrap"> <a href="https://blog.csdn.net/chinoukin/article/details/79005596" title="为什么有了事务这东西，还需要乐观锁悲观锁？" style="background:transparent; color:rgb(51,90,164)" rel="nofollow">为什么有了事务这东西，还需要乐观锁悲观锁？</a></li> 
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     <span style="">回复于： 2011-09-02 09:27:05</span> 
     <span style="float:right; margin-left:50px"><a href="https://bbs.csdn.net/topics/370165246#post-380591708" style="background:transparent; color:rgb(102,102,102); margin:0px 2px" rel="nofollow">#9</a>&nbsp;得分：5</span> 
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    <div style="list-style:none; margin:0px; padding:16px; border:none; outline:none 0px; word-wrap:break-word; overflow:hidden; font-size:14px; line-height:28px; width:758px">   引用&nbsp;11&nbsp;楼&nbsp;fanxiaohe123&nbsp;的回复: 
     <blockquote style="padding:0px; margin:0px; font-size:14px; border:none; list-style:none; outline:none 0px">
       乐观锁用白话一点说：先改，改了再说，要是别人修改过了我就不再改了，要改也得重新查再改 
      <br style=""> 一般是表共享，行排他 
      <br style=""> 最好是乐观锁，因为效率，但当C/S结构时，悲观的好。 
      <br style=""> （都是自己的个人看法） 
     </blockquote> 在oracle数据库里，举个乐观锁的例子？ 
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      <li style="list-style:none; margin:0px; padding:0px; border:none; outline:none 0px; width:820px; line-height:25px; overflow:hidden; text-overflow:ellipsis; white-space:nowrap"> <a href="https://blog.csdn.net/shangruo/article/details/52777596" title="java 5多线程乐观锁CAS之于悲观锁synchronized" style="background:transparent; color:rgb(51,90,164)" rel="nofollow">java 5多线程乐观锁CAS之于悲观锁synchronized</a></li> 
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     <span style="">回复于： 2011-09-07 08:03:53</span> 
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      乐观锁，大多是基于数据版本（&nbsp;Version&nbsp;）记录机制实现。 
     <br style=""> 数据版本即为数据增加一个版本标识，在基于数据库表的版本解决方案中，一般是通过为数据库表增加一个&nbsp;“version”&nbsp;字段来实现。 
     <br style=""> 读取出数据时，将此版本号一同读出，之后更新时，对此版本号加一。此时，将提交数据的版本数据与数据库表对应记录的当前版本信息进行比对，如果提交的数据版本号大于数据库表当前版本号，则予以更新，否则认为是过期数据。 
     <br style=""> 对于上面修改用户帐户信息的例子而言，假设数据库中帐户信息表中有一个version&nbsp;字段，当前值为&nbsp;1&nbsp;；而当前帐户余额字段（&nbsp;balance&nbsp;）为&nbsp;$100&nbsp;。 
     <br style=""> 1&nbsp;操作员&nbsp;A&nbsp;此时将其读出（&nbsp;version=1&nbsp;），并从其帐户余额中扣除&nbsp;$50（&nbsp;$100-$50&nbsp;）。 
     <br style=""> 2&nbsp;在操作员&nbsp;A&nbsp;操作的过程中，操作员&nbsp;B&nbsp;也读入此用户信息（&nbsp;version=1&nbsp;），并从其帐户余额中扣除&nbsp;$20&nbsp;（&nbsp;$100-$20&nbsp;）。 
     <br style=""> 3&nbsp;操作员&nbsp;A&nbsp;完成了修改工作，将数据版本号加一（&nbsp;version=2&nbsp;），连同帐户扣除后余额（&nbsp;balance=$50&nbsp;），提交至数据库更新，此时由于提交数据版本大于数据库记录当前版本，数据被更新，数据库记录&nbsp;version&nbsp;更新为&nbsp;2&nbsp;。 
     <br style=""> 4&nbsp;操作员&nbsp;B&nbsp;完成了操作，也将版本号加一（&nbsp;version=2&nbsp;）试图向数据库提交数据（&nbsp;balance=$80&nbsp;），但此时比对数据库记录版本时发现，操作员&nbsp;B&nbsp;提交的数据版本号为&nbsp;2&nbsp;，数据库记录当前版本也为&nbsp;2&nbsp;，不满足&nbsp;“&nbsp;提交版本必须大于记录当前版本才能执行更新&nbsp;“&nbsp;的乐观锁策略，因此，操作员&nbsp;B&nbsp;的提交被驳回。 
     <br style=""> 这样，就避免了操作员&nbsp;B&nbsp;用基于&nbsp;version=1&nbsp;的旧数据修改的结果覆盖操作员&nbsp;A&nbsp;的操作结果的可能。 
     <br style=""> 乐观锁机制避免了长事务中的数据库加锁开销，大大提升了大并发量下的系统整体性能表现。 
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      <li style="list-style:none; margin:0px; padding:0px; border:none; outline:none 0px; width:820px; line-height:25px; overflow:hidden; text-overflow:ellipsis; white-space:nowrap"> <a href="https://blog.csdn.net/qq_27550755/article/details/50071823" title="SSH：Hibernate框架（悲观锁和乐观锁解决hibernate并发 ）" style="background:transparent; color:rgb(51,90,164)" rel="nofollow">SSH：Hibernate框架（悲观锁和乐观锁解决hibernate并发 ）</a></li> 
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     <span style="">回复于： 2011-09-07 09:17:40</span> 
     <span style="float:right; margin-left:50px"><a href="https://bbs.csdn.net/topics/370165246#post-380676003" style="background:transparent; color:rgb(102,102,102); margin:0px 2px" rel="nofollow">#13</a>&nbsp;得分：6</span> 
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      补充一下 
     <br style=""> 在乐观锁中，常用的实现方法有3种。 
     <br style=""> 
     <br style=""> [1]第一种就是在数据取得的时候把整个数据都copy到应用中，在进行提交的时候比对当前数据库中的数据和开始的时候更新前取得的数据。当发现两个数据一模一样以后，就表示没有冲突可以提交，否则则是并发冲突，需要去用业务逻辑进行解决。 
     <br style=""> 
     <br style=""> [2]第二种乐观锁的做法就是采用版本戳，就是我上边说的那个[#13楼&nbsp;] 
     <br style=""> [3]第三种做法和第二种做法有点类似，就是也新增一个Table的Column，不过这次这个column是采用timestamp型，存储数据最后更新的时间。在Oracle9i以后可以采用新的数据类型，也就是timestamp&nbsp;with&nbsp;time&nbsp;zone类型来做时间戳。这种Timestamp的数据精度在Oracle的时间类型中是最高的，精确到微秒(还没与到纳秒的级别)，一般来说，加上数据库处理时间和人的思考动作时间，微秒级别是非常非常够了，其实只要精确到毫秒甚至秒都应该没有什么问题。和刚才的版本戳类似，也是在更新提交的时候检查当前数据库中数据的时间戳和自己更新前取到的时间戳进行对比，如果一致则OK，否则就是版本冲突。如果不想把代码写在程序中或者由于别的原因无法把代码写在现有的程序中，也可以把这个时间戳乐观锁逻辑写在Trigger或者存储过程中。 
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      <li style="list-style:none; margin:0px; padding:0px; border:none; outline:none 0px; width:820px; line-height:25px; overflow:hidden; text-overflow:ellipsis; white-space:nowrap"> <a href="https://blog.csdn.net/ydk888888/article/details/78625333" title="Java乐观锁悲观锁、synchronized,重入锁 (ReentrantLock)处理并发（互斥同步、非互斥同步）" style="background:transparent; color:rgb(51,90,164)" rel="nofollow">Java乐观锁悲观锁、synchronized,重入锁 (ReentrantLock)处理并发（互斥同步、非互斥同步）</a></li> 
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     <span style="">回复于： 2011-09-07 09:29:31</span> 
     <span style="float:right; margin-left:50px"><a href="https://bbs.csdn.net/topics/370165246#post-380676441" style="background:transparent; color:rgb(102,102,102); margin:0px 2px" rel="nofollow">#14</a>&nbsp;得分：6</span> 
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