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

This sounds like a problem that might be better solved by a non-relational solution, like MongoDB or CouchDB.

They both allow for dynamic schema expansion while allowing you to maintain the tuple integrity you seek.

I agree with Bill Karwin, the EAV model is not a performant approach for you. Using name-value pairs in a relational system is not intrinsically bad, but only works well when the name-value pair make a complete tuple of information. When using it forces you to dynamically reconstruct a table at run-time, all kinds of things start to get hard. Querying becomes an exercise in pivot maintenance or forces you to push the tuple reconstruction up into the object layer.

You can't determine whether a null or missing value is a valid entry or lack of entry without embedding schema rules in your object layer.

You lose the ability to efficiently manage your schema. Is a 100-character varchar the right type for the "value" field? 200-characters? Should it be nvarchar instead? It can be a hard trade-off and one that ends with you having to place artificial limits on the dynamic nature of your set. Something like "you can only have x user-defined fields and each can only be y characters long.

With a document-oriented solution, like MongoDB or CouchDB, you maintain all attributes associated with a user within a single tuple. Since joins are not an issue, life is happy, as neither of these two does well with joins, despite the hype. Your users can define as many attributes as they want (or you will allow) at lengths that don't get hard to manage until you reach about 4MB.

If you have data that requires ACID-level integrity, you might consider splitting the solution, with the high-integrity data living in your relational database and the dynamic data living in a non-relational store.