database relationships

问题:

does setting up proper relationships in a database help with anything else other than data integrity?

do they improve or hinder performance?

回答1:

I'd have to say that proper relationships will help people to understand the data (or the intention of the data) better than if omitting them, especially as the overall cost is quite low in maintaining them.

Their presence doesn't hinder performance except in terms of architecture (as others have pointed out, data integrity will occasionally cause foreign key violations which may have some effect) but IMHO is outweighed by the many benefits (if used correctly).

I know you weren't asking whether to use FKs or not, but I thought I'd just add a couple of viewpoints about why to use them (and have to deal with the consequences):

There are other considerations too, such as if you ever plan to use an ORM (perhaps later on) you'll require foreign keys. They can also be very helpful for ETL/Data Import and Export and later for reporting and data warehousing.

It's also helpful if other applications will make use of the schema - since Foreign Keys implement a basic business logic. So your application (and any others) only need to be aware of the relationships (and honour them). It'll keep the data consistent and most likely reduce the number of data errors in any consuming applications.

Lastly, it gives you a pretty decent hint as to where to put indexes - since it's likely you'll lookup table data by an FK value.

回答2:

As long as you have the obvious indexes in place corresponding to the foreign keys, there should be no perceptible negative effect on performance. It's one of the more foolproof database features you have to work with.

回答3:

It neither helps nor hurts performance in any significant way. The only hindrance is the check for integrity when inserting/updating/deleting.

Foreign keys are an important part of database design because they ensure consistency. You should use them because it offers the lowest level of protection against data screw ups that can wreck your applications. Another benefit is that database tools (visualization/analysis/code generation) use foreign keys to relate data.

回答4:

Do relationships in databases improve or hinder performance?

Like any tool in your toolbox, the results you'll get depend on how you use it. Properly specified relationships and a well-designed logical database can be an enormous boon to performance -- consider the difference between searching through normalized and denormalized data, for example.

回答5:

Depending on your database engine, relationships defined through foreign key constraints can benefit performance. The constraint allows the engine to make certain assumptions about the existence of data in tables on the parent side of the key.

A brief explanation for MS SQL Server can be found at http://www.microsoft.com/technet/abouttn/flash/tips/tips_122104.mspx. I don't know about other engines, but the concept would make sense in other platforms.

回答6:

Relationships in the data exist whether you declare them or not. Declaring and enforcing the relationships via FK constraints will prevent certain kinds of errors in the data, at a small cost of checking data when inserts/updates/deletes occur.

Declaring cascading deletes via relationships helps prevent certain kinds of errors when deleting data.

Knowing the relationships helps to make flexible and correct use of the data when forming queries.

Designing the tables well can make the relationships more obvious and more useful. Using relationships in the data is the primary power behind using relational databases in the first place.

回答7:

About impact on performance: In my experience with MS Access 2003, if you have a multi-user application and use Relationships to enforce a lot of referential integrity, you can take a big hit in terms of response time for the end-user.

There are different ways to take care of enforcing referential integrity. I decided to take out some rules in Relationships, build more enforcement into the front-end and live with some loss of RI. Of course in the multi-user environment, you want to be very careful with that bit of liberty.

回答8:

In my experience building performance-sensitive databases, Foreign Keys hurt performance pretty significantly, since they have to be checked every time the referring record is inserted/updated or master record is deleted. If you need a proof, just look at the execution plan.

I still keep them for documentation and for tools to use but I usually disable them, especially in high-performance systems where access to DB is only through the application layer.