数据库字段中的XMLSerialized对象。 这是好的设计吗？

Question

Suppose i have one table that holds Blogs. The schema looks like :

ID (int)| Title (varchar 50) | Value (longtext) | Images (longtext)| ....

In the field Images i store an XML Serialized List of images that are associated with the blog.

Should i use another table for this purpose?

Answer 1

Yes, you should put the images in another table. Having several values in the same field indicates denormalized data and makes it hard to work with the database.

As with all rules, there are exceptions where it makes sense to put XML with multiple values in one field in the database. The first rule is that:

The data should always read/written together. No need to read or update just one of the values.

If that is fulfilled, there can be a number of reasons to put the data together in one field:

• Storage efficiency, if space has proved to be a problem.
• Retrieval efficiency, if performance has proved to be a problem.
• Schema flexilibity; where one XML field can eliminate tens or hundreds of different tables.

Answer 2

I would certainly use another table. If you use XML, what happens when you need to go through and update the references to all images? (Would you just rather do an Update blog_images Set ... , or parse through the XML for each row, make the update, then re-generate the updated XML for each?

Answer 3

Well, it is a bit "inner platform", but it will work. A separate table would allow better image querying, although on some RDBMS platforms this could also be achieved via an XML-type column and SQL/XML.

If this data only has to be opaque storage, then maybe. However, keep in mind you'll generally have to bring back the entire XML to the app-tier to do anything interesting with it (or: depending on platform, use SQL/XML, but I advise against this, as the DB isn't the place to do such processing in most cases).

My advice in all other cases: separate table.

Answer 4

That depends on whether you'd need to query on the actual image data itself. If you see a possible need to query on certain images, or images with certain attributes, then it would probably be best to store that image data in a different way.

Otherwise, leave it the way it is.

But remember, only include the fields in your SELECT when you need them.

Answer 5

Should i use another table for this purpose?

Not necessarily. You just have to ensure that you are not selecting the images field in your queries when you don't need it. But if you wanted to denormalize your schema you could use another table and when you need the images perform a join .