oracle允许未提交的读取选项？

发布时间：2020-12-12 13:48:22 所属栏目：百科来源：网络整理

导读：在db2中,使用’with ur’子句的查询允许查询为未提交的读取,mysql中的’with nolock’子句也是如此.在oracle里有没有这样的选择？如果不是为什么？汤姆提供了一个很好的答案： On Transaction Isolation Levels 他说： The READ UNCOMMITTED isolation leve

在db2中,使用’with ur’子句的查询允许查询为未提交的读取,mysql中的’with nolock’子句也是如此.在oracle里有没有这样的选择？如果不是为什么？汤姆提供了一个很好的答案： On Transaction Isolation Levels

他说：

The READ UNCOMMITTED
isolation level allows dirty reads.
Oracle Database doesn’t use dirty
reads,nor does it even allow them.
The basic goal of a READ UNCOMMITTED
isolation level is to provide a
standards-based definition that allows
for nonblocking reads.

…

Now,a database that allowed a dirty
read … not only does it return the
wrong answer,but also it returns … [an answer] … that never existed in the table.
In a multiuser database,a dirty read
can be a dangerous feature.
Personally,I’ve never seen the
usefulness of it…

The point here is that dirty read is not a feature; rather,it’s a liability. In Oracle Database,it’s just not needed. You get all of the advantages of a dirty read—no blocking—without any of the incorrect results.

（编辑：李大同）

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