更大的问题是:你会戴着眼罩开车吗?如果你开发一个没有参考约束的系统就是这样。请记住，业务需求会改变，应用程序设计会改变，代码中相应的逻辑假设也会改变，逻辑本身也会被重构，等等。一般来说，数据库中的约束是在当代逻辑假设下放置的，对于特定的逻辑断言和假设集似乎是正确的。

在应用程序的整个生命周期中，引用检查和数据检查约束控制通过应用程序收集的数据，特别是当新需求驱动逻辑应用程序更改时。

从实时事务处理系统的角度来看，外键本身不会“提高性能”，也不会显著“降低性能”。然而，在大容量“批处理”系统中，约束检查存在一个聚合代价。这就是实时和批量事务处理的区别;批处理——通过约束检查，按顺序处理的批处理的总成本会对性能造成影响。

在一个设计良好的系统中，数据一致性检查将在处理批处理之前完成(然而，这里也有相关的成本);因此，加载时不需要外键约束检查。事实上，所有的约束，包括外键，都应该暂时禁用，直到批处理完成。

QUERY性能——如果表是在外键上连接的，那么要认识到外键列不是索引的事实(尽管根据定义，各自的主键是索引的)。通过索引一个外键，也就是索引任何键，在索引上连接表有助于提高性能，而不是通过连接一个带外键约束的非索引键。

Changing subjects, if a database is just supporting website display/rendering content/etc and recording clicks, then a database with full constraints on all tables is over kill for such purposes. Think about it. Most websites don’t even use a database for such. For similar requirements, where data is just being recorded and not referenced per say, use an in-memory database, which does not have constraints. This doesn’t mean that there is no data model, yes logical model, but no physical data model.

我经常使用它们，但后来我为金融系统制作数据库。数据库是应用程序的关键部分。如果金融数据库中的数据不是完全准确的，那么无论你在代码/前端设计上投入多少精力都没有意义。你只是在浪费时间。

还有一个事实是，多个系统通常需要直接与数据库接口-从其他系统只是读取数据(Crystal Reports)到系统插入数据(不一定使用我设计的API;它可能是由一个刚刚发现VBScript并拥有SQL框SA密码的愚蠢的经理编写的)。如果数据库不能像白痴一样证明它可能是，好吧，再见数据库。

如果您的数据很重要，那么可以使用外键，创建一套存储过程来与数据交互，并创建最强大的DB。如果您的数据不重要，那么为什么要开始创建数据库呢?

我同意德米特里的回答，说得很好。

For those who are worried about the performance overhead FK's often bring, there's a way (in Oracle) you can get the query optimiser advantage of the FK constraint without the cost overhead of constraint validation during insert, delete or update. That is to create the FK constraint with the attributes RELY DISABLE NOVALIDATE. This means the query optimiser ASSUMES that the constraint has been enforced when building queries, without the database actually enforcing the constraint. You have to be very careful here to take the responsibility when you populate a table with an FK constraint like this to make absolutely sure you don't have data in your FK column(s) that violate the constraint, as if you do so you could get unreliable results from queries that involve the table this FK constraint is on.

我通常在数据集市模式中的某些表上使用这种策略，但在集成登台模式中不使用。我要确保复制数据的表已经强制执行了相同的约束，或者ETL例程强制执行了该约束。

使用外键的原因:

you won't get Orphaned Rows you can get nice "on delete cascade" behavior, automatically cleaning up tables knowing about the relationships between tables in the database helps the Optimizer plan your queries for most efficient execution, since it is able to get better estimates on join cardinality. FKs give a pretty big hint on what statistics are most important to collect on the database, which in turn leads to better performance they enable all kinds of auto-generated support -- ORMs can generate themselves, visualization tools will be able to create nice schema layouts for you, etc. someone new to the project will get into the flow of things faster since otherwise implicit relationships are explicitly documented

不使用外键的原因:

you are making the DB work extra on every CRUD operation because it has to check FK consistency. This can be a big cost if you have a lot of churn by enforcing relationships, FKs specify an order in which you have to add/delete things, which can lead to refusal by the DB to do what you want. (Granted, in such cases, what you are trying to do is create an Orphaned Row, and that's not usually a good thing). This is especially painful when you are doing large batch updates, and you load up one table before another, with the second table creating consistent state (but should you be doing that sort of thing if there is a possibility that the second load fails and your database is now inconsistent?). sometimes you know beforehand your data is going to be dirty, you accept that, and you want the DB to accept it you are just being lazy :-)

我认为(我不确定!)大多数已建立的数据库都提供了一种指定外键的方法，这种方法不是强制的，只是一些元数据。由于不强制执行消除了不使用fk的所有理由，如果第二部分中的任何理由适用，您可能应该走那条路。

它们会使删除记录变得更加麻烦——当其他表中存在外键违反约束的记录时，您就不能删除“主”记录。可以使用触发器进行级联删除。

如果不明智地选择主键，则更改该值将变得更加复杂。例如，如果我有我的“客户”表的PK作为人的名字，并使该键在“订单”表中为FK，如果客户想更改他的名字，那么这是一个巨大的痛苦……但这只是粗制滥造的数据库设计。

我相信使用火密钥的优点大于任何假定的缺点。

Wowowo... Answers everywhere. Actually this is the most complicated topic I have ever encountered. I use FKs when they are needed but on production environment I rarely use them. Here is my whys I rarely use the Fks: 1. Most of the time I am dealing with huge data on small server to improve performance I need to remove the FKs. Because when you have FKs and you do Create, Update or Delete the RDBMS first check if there no constraint violation and if you have huge DB that could be something fatal 2. Sometimes I need to import data from others places and because I am not too sure of how well structured they are, I simply drop the FKs. 3. In case you are dealing with multiple DBs and having reference key in an other DB will not go well(as for now) until you remove the FKs (cross database relations) 4. They was also a case when you write an application which will seat on whatever RDBMS or you want your DB to be exported and imported in any RDBMS system in this case each specific RDBMS system has his own way of dealing with FKs and you will probably be obliged to drop the use of FKs. 5. If you user RDBMS platform (ORMs) you know that some of them offer their own mapping depending on the solution and technicality their offer and you don't care about creating the tables and their FKs. 6. Before the last point will be knowledge to deal with DB that has FKs and the knowledge to write an application that does all the Job without the need of FK 7. Lastly as I started saying it all depend on your scenario, in case knowledge is not a barrier. You will always want to run the best of the best you can get!

谢谢大家!