UDF in check constraint -- downside? I have a table where all actions affect single rows (insert and update, no delete's but if there were, they would be individual as well).  I'm considering using a UDF in a check constraint so I can make sure that a field value is only used in combination with another value (two FK's, the first time A=1 is used, B=x becomes fixed, so that if x = 4 then A=1 can never be matched with B=3 or anything except 4).

What are the downsides of doing this, other than the neligible additional time spent when inserting/updating the row?

UPDATE: The application doesn't provide a mechanism for changing either column A or B after the row has been created.
You can store A and B columns in a separate table. Make sure your table has a primary key on A, and a unique constraint on (A,B) Refer to (A,B) from your table. The primary key on A will guarantee only one B per A. The foreign key without ON UPDATE CASCADE will make sure B does not change as long as the row is referred from the child table.