From my experience its always better to avoid using FKs in Database Critical Applications. I would not disagree with guys here who say FKs is a good practice but its not practical where the database is huge and has huge CRUD operations/sec. I can share without naming ... one of the biggest investment bank of doesn't have a single FK in databases. These constrains are handled by programmers while creating applications involving DB. The basic reason is when ever a new CRUD is done it has to effect multiple tables and verify for each inserts/updates, though this won't be a big issue for queries affecting single rows but it does create a huge latency when you deal with batch processing which any big bank has to do as daily tasks.

最好避免fk，但它的风险必须由程序员来处理。

像许多事情一样，这是一种权衡。这是一个你想在哪里进行验证数据完整性的工作的问题:

(1)使用外键(单点配置为一个表，功能已经实现，经过测试，证明有效)

(2)把它留给数据库的用户(可能多个用户/应用程序更新同一个表)，这意味着更多潜在的故障点和测试的复杂性)。

数据库执行(2)更有效，使用(1)更容易维护，风险更小。

在DB2中，如果使用mqt(物化查询表)，优化器需要外键约束才能为任何给定的查询选择正确的计划。由于元数据包含基数信息，优化器会大量使用元数据来决定是否使用MQT。

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。

我听到的观点是前端应该有这些业务规则。当您一开始就不应该允许任何破坏约束的插入时，外键会“增加不必要的开销”。我同意吗?不，但我一直都是这么听说的。

编辑:我猜他指的是外键约束，而不是外键这个概念。

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!