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Read Committed Isolation (327)

原创 ERP 作者:tsinglee 时间:2007-12-08 13:42:45 0 删除 编辑

For many applications, read committed is the most appropriate isolation level. Read
committed isolation can provide considerably more concurrency with a somewhat
increased risk of inconsistent results due to phantoms and non-repeatable reads for
some transactions.

Many high-performance environments with high transaction arrival rates require more
throughput and faster response times than can be achieved with serializable isolation.
Other environments that supports users with a very low transaction arrival rate also
face very low risk of incorrect results due to phantoms and nonrepeatable reads. Read
committed isolation is suitable for both of these environments.

Oracle read committed isolation provides transaction set consistency for every query.
That is, every query sees data in a consistent state. Therefore, read committed isolation
will suffice for many applications that might require a higher degree of isolation if run
on other database management systems that do not use multiversion concurrency
control.

Read committed isolation mode does not require application logic to trap the Cannot
serialize access error and loop back to restart a transaction. In most applications,
few transactions have a functional need to issue the same query twice, so for many
applications protection against phantoms and non-repeatable reads is not important.

Therefore many developers choose read committed to avoid the need to write such
error checking and retry code in each transaction.

已提交读隔离
1.
已提交读取隔离能够最大限度地保证数据并发性,但在某些事务中可能会出现不可重复读取或幻象读取
的风险
2.
在对性能要求较高的系统中,为了应对较高的事务到来率,系统需要提供更大的事务吞吐量和更快的响
应速度,此时采用串行化隔离可能难以实现.还有一类系统,其事务到来率较低,出现不可重复读取或
幻象读取的风险也较低. 以上两种系统均适合采用已提交读取隔离

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