Question

I have a table that contains all products. There are 3 distinct types of products so these have their own tables, lets say ProductType1, ProductType2, ProductType3.

There is a 1-1 relationship between Products and ProductType(n) on ProductId, but to further constraint the child tables there is an additional relationship using a ProductId, ProductTypeId in Products, and a ProductId, ComputedProductTypeId in each of the other tables.

This ensures that a product can only be added to a single matching ProductType table.

The question is this. As there is already a relationship between the 2 tables on ProductId, rather than using an index for the FK, can I get away with a unique key to constrain the relationship, or will this cause performance issues?

Products

PK ProductId
FK ProductId, ProductTypeId
 ^
*Add an index for this or unique key constraint?*

ProductType(n)

PK ProductId
FK ProductID, ComputedProductTypeId (fixed int)

Answer 1

Creating an index will be better approach.
If you want to delete entries from your master table, SQL server looks for FK relations if any exists. So Creating Index on your composite key (which includes FK) will speed up the process.