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SQL隔离级别，读写锁

[英]Sql isolation levels, Read and Write locks

原文 2013-05-26 14:16:45 5 3 sql/ transactions/ isolation-level

A bit lame question but I got confused... Difference between isolation levels as far as I understood is how they managed their locks ( http://en.wikipedia.org/wiki/Isolation_(database_systems) ). 有点la脚的问题，但我感到困惑...据我所知，隔离级别之间的区别是它们如何管理锁（ http://en.wikipedia.org/wiki/Isolation_(database_systems））。 So as mentioned in the article there are Read, Write and Range locks but there is no definition what they are itself. 因此，如文章中所述，存在读取，写入和范围锁定，但没有定义它们本身是什么。 What are you allowed to do and what not. 你被允许做什么和不可以做什么。 When I googled for it there was nothing concrete and instead I got confused with new terms like Pessimistic Lock an Optimistic Lock, Exclusive lock, Gap lock and so on. 当我搜索它时，并没有什么具体的含义，相反，我对诸如悲观锁，乐观锁，互斥锁，间隙锁等新术语感到困惑。 I'd be pleased if someone give me a short overview and maybe point me a good bunch materials to enlighten myself. 如果有人给我一个简短的概述，或者为我指出很多启发我的东西，我会很高兴。

My initial question which started the research of isolation levels was: 我最初的问题开始研究隔离级别：

What happens when I have concurrent inserts (different users of web app) into one table when my transactions isolation level is READ_COMMITED. 当我的事务隔离级别为READ_COMMITED时，将并发插入（Web应用程序的不同用户）插入一个表时会发生什么。 Is the whole table locked or not? 整个桌子都上锁了吗？ Or generally what happens down there :) ? 还是一般来说，那里发生了什么：）？

Thanks in advance ! 提前致谢！

3 个解决方案

What happens when I have concurrent inserts (different users of web app) into one table when my transactions isolation level is READ_COMMITED. 当我的事务隔离级别为READ_COMMITED时，将并发插入（Web应用程序的不同用户）插入一个表时会发生什么。

"Read committed" means that other sessions cannot see the newly inserted row until its transaction is committed. “读取已提交”表示其他会话在提交事务之前无法看到新插入的行。 A SQL statement that runs without an explicit transaction is wrapped in an implicit one, so "read committed" affects all inserts. 在没有显式事务的情况下运行的SQL语句被包装在隐式事务中，因此“已读提交”会影响所有插入。

Some databases implement "read committed" with locks. 一些数据库使用锁定来实现“已读读”。 For example, a read lock can be placed on the inserted row, preventing other tractions from reading it. 例如，可以在插入的行上放置一个读锁，以防止其他牵引力读取它。 Other databases, like Oracle, use multiversion concurrency control . 其他数据库（例如Oracle）使用多版本并发控制。 That means they can represent a version of the database before the insert. 这意味着它们可以代表插入之前的数据库版本。 This allows them to implement "read committed" without locks. 这使他们可以实现不带锁的“读取已提交”。