How to design a database for User Defined Fields?

Question

My requirements are:

• Need to be able to dynamically add User-Defined fields of any data type
• Need to be able to query UDFs quickly
• Need to be

Answer 1

I have written about this problem a lot. The most common solution is the Entity-Attribute-Value antipattern, which is similar to what you describe in your option #3. Avoid this design like the plague.

What I use for this solution when I need truly dynamic custom fields is to store them in a blob of XML, so I can add new fields at any time. But to make it speedy, also create additional tables for each field you need to search or sort on (you don't a table per field--just a table per searchable field). This is sometimes called an inverted index design.

You can read an interesting article from 2009 about this solution here: http://backchannel.org/blog/friendfeed-schemaless-mysql

Or you can use a document-oriented database, where it's expected that you have custom fields per document. I'd choose Solr.