为什么复合键在hibernate不鼓励？ [英] Why are composite keys discouraged in hibernate?

问题描述

这是来自 Hibernate官方教程 ：

有一个替代< composite-id> 到具有复合键的遗留数据。

There is an alternative <composite-id> declaration that allows access to legacy data with composite keys. Its use is strongly discouraged for anything else.

为什么不鼓励复合键？我正在考虑使用一个3列表，其中所有的列都是外键，并一起形成一个主键，这是一个有意义的关系，在我的模型。我不明白为什么这是一个坏主意，特别是我将使用他们的索引。

Why are composite keys discouraged? I am considering using a 3-column table where all of the columns are foreign keys and together form a primary key that is a meaningful relationship in my model. I don't see why this is a bad idea, espicially that I will be using an index on them.

有什么替代方法？创建其他自动生成的列并将其用作主键？我仍然需要查询我的3列吗？

What's the alternative? Create an additional automatically generated column and use it as a primary key? I still need to query my 3 columns anyways!?

总之，为什么这个语句是真的？

In short, why is this statement true? and what's the better alternative?

推荐答案

他们不鼓励他们有几个原因：

They discourage them for several reasons:

• 使用起来很麻烦。每次您需要引用一个对象（或行）时，例如在您的Web应用程序中，您需要传递3个参数，而不只是一个。


• 它们效率低下。


• 它们会导致错误：开发人员不可避免地不正确地实现主键类的equals和hashCode方法，而不是简单地对整数进行散列。或者它们使它变得可变，并且一旦存储在HashSet或HashMap中就修改它们的值


• 它们污染了模式。如果另一个表需要引用这个3列表，它将需要有一个3列，而不是只有一个作为外键。现在假设你遵循相同的设计，并使这个新表的主键的这个3列外键部分，你会很快有一个4列的主键，然后在下表中的5列PK等。

• they're cumbersome to use. Each time you need to reference an object (or row), for eexample in your web application, you need to pass 3 parameters instead of just one.
• they're inefficient. Instead of simply hashing an integer, the database needs to hash a composite of 3 columns.
• they lead to bugs: developers inevitably implement the equals and hashCode methods of the primary key class incorrectly. Or they make it mutable, and modify their value once stored in a HashSet or HashMap
• they pollute the schema. If another table needs to reference this 3-column table, it will need to have a 3 columns instead of just one as a foreign key. Now suppose you follow the same design and make this 3-column foreign key part of the primary key of this new table, you'll quickly have a 4-column primary key, and then a 5-column PK in the next table, etc. etc., leading to duplication of data, and a dirty schema.

另一种方法是使用单列，生成的主键，除了其他三列。如果要使三列的元组唯一，则使用唯一约束。

The alternative is to have a single-column, auto-generated primary key, in addition to the other three columns. If you want to make the tuple of three columns unique, then use a unique constraint.

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