选择N + 1是否比渴望加入更好? [英] Is select N + 1 ever better than eager join?

问题描述

我正在使用nhibernate探查器，并在一些地方给了我选择n + 1"警告.

i am using nhibernate profiler and seeing a few places where it is giving me a "Select n + 1" warning.

一个明显的解决方法是进行一些前期热切的联接，但是让我思考的是，在某些情况下，与选择可能要加载许多您从未实际使用的数据的联接相比，使用Select N +1的情况是否更简单或更快速

The obvious fix is to make some upfront eager joins but it got me thinking if in certain cases its actually simpler or faster to live with Select N + 1 compared to doing joins that may be load lots of data that you never actually use.

推荐答案

您的想法是正确的.例如，在某些情况下，所有根实体都引用另一个实体的一个或两个实例.进行2或3个小选择而不是非规范化的选择(即使用联接)可能会更快.

Your thinking is correct. In some cases, for example, all the root entities reference one or two instances of another entity. It might be faster to do 2 or 3 small selects instead of a denormalized one (i.e. with joins)

几乎在所有情况下都有一种使此操作更方便的方法:batch-size.如果您在实体和集合中都将此属性设置为通常的页面大小，则将获得恒定数量的小选择(每种实体类型一个).

There is a way to make this convenient in almost all cases: batch-size. If you set this attribute in both the entities and the collections to your usual page size, you'll get a constant number of small selects (one per entity type).

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